1) The value of studying the response of children to graded exercise is discussed. It is pointed out that testing the cardiovascular and respiratory systems under load often provides more accurate and more useful information than can be obtained at rest. 2) It is noted that little is known from previous work about the changes of cardiac output and stroke volume in children during exercise because of the inappropriateness of standard methods of measurements, which involve cardiac and arterial catheterisation, The available data on changes in heart rate and ventilation in children is reviewed. 3) The historical development of the Indirect (CO2) Fick method for measuring cardiac output is described, and the suitability of this technique for studies in children is noted because it does not require cardiac catheterisation. An account is given of the apparatus and techniques developed in the present study to measure the cardiac and respiratory responses of children to exercise. The rebreathing method used to measure mixed venous PCO2, and the method of measuring arterialised ear lobe blood gases during exercise are detailed. d) Two types of exercise test were used to study the responses of the children in the present study. The first test was a simple progressive exercise test in which a guide was obtained to the limit of physical working capacity by increasing the work load on a cycle ergometer every minute until the subject was unable to continue. The increments of work were graded according to height. The second test, a steady state exercise test, was performed at 1/3 and 2/3 of the maximum work load completed in the simple progressive test after a rest period. There was no pause between work loads. 5) A computer program was written to perform the cal culation of the results of exercise tests based on the Indirect (CO2) Fick method. The program calculated the results for each set of input data and made any necessary corrections. It constructed a graph relating cardiac output, dead space and arterial PCO2, for the individual work load, based on the interrelationship of the Fick and Bohr equation. The derivation of the equations and the principles of the computer program are described. 6) The computer-produced graphs were used to explore the effect of different estimates of arterial PCO2 on the calculation of cardiac output. It was found that the cardiac output on exercise was largely independent of the method used to estimate arterial PCO2,, but results at rest were unreliable. In children with normal lungs, it was concluded that the most satisfactory method was to calculate arterial PCO2 by assuming dead space to be normal. 7) Worked examples are given to illustrate the use of the methods described in healthy and sick children. The examples were chosen to show various problems of interpretation which may arise. 8) The results of a study in 57 normal boys and 60 normal girls aged 6 to 16 years are described. The maximum work load completed in the simple progressive exercise test was directly proportional to height, with boys achieving rather higher loads than girls. The maximum heart rate was independent of size and sex. A successful steady state exercise test was completed in 905 of children. Growth had little effect on oxygen consumption, minute ventilation or cardiac output for any given work level, and there were no important sex differences. Tidal volume, dead space and stroke volume increased and heart rate decreased with increasing size for any given work load. Girls had smaller stroke volume and higher heart rates than boys of similar size. Blood lactate at any given fraction of the physical working capacity was the same as the values reported in adults. 9) The ability to apply the tests described to sick children is emphasized with reference to preliminary studies in 100 children with various types of heart and lung disease. It is noted that ear lobe blood PCO2 may be used in place of arterial PCO2, in the cal culations, when lung disease precludes the assumption that dead space is normal. Very characteristic patterns of abnormality have been noted in certain disease such as cystic fibrosis, which explain the effort intolerance and help to quantitate the extent of the abnormality. The value of exercise testing in helping to establish normality in the child with an innocent heart murmur or with symptoms lacking an organic basis is noted.