1.1. Definition (a) "Infrasound is a mechanical disturbance, propagated on an elastic medium, of frequency too low to be capable of exciting the sensation of hearing". (British Standard 661) (b) In this work the upper limit of the infrasonic region will generally be taken as 20Hz. Propagation is in air unless otherwise stated. 1.2. The literature on the effects of infrasound on both man and animals is reviewed in Chapter 2. In Chapter 3 the general requirements and possible sources of infrasound are discussed, the final apparatus is described together with its calibration and performance. Chapters 4 and 5 deal with the methods used to detect both subjective and objective effects evoked by the stimulus and the preliminary experiments performed to test the feasibility of the project. The experiments performed with the system in examining the physiological effects of monaural, binaural and whole body stimulation are discussed in detail in Chapters 6 and 7. The physiological background to the experiments is discussed in Chapter 8. Finally in chapters 9 and 10 the results are discussed and possible mechanisms for the production of the effects postulated.