Five pot and five field experiments were conducted during the period 1978-1981 to investigate the effects of early applications of the growth retardant, CCC, and the growth promoter, GA₃, on tillering, ear production and grain yield of both spring (7 experiments) and winter (3 experiments) barley. The pre-tillering applications of low concentrations of both CCC (1000 ppm) and GA₃ (20 ppm) resulted in consistent increases in total shoot and ear production per plant in the pot experiments. Similar treatments in the field led to consistent increases in ear production per unit area. These increases in ear production were not always accompanied by increased tillering and were due mainly to higher tillering efficiency in the treated plants. The CCC-induced increases in ear numbers were attributable to a greater proportion of secondary tillers particularly by the Tl(1) tiller surviving and producing ears in spring barley and to a similar influence on the late primary tiller, T3 in winter barley. In contrast an enhanced ear production by the primary tillers, T1, T2 and T3 was the source of the extra ears seen in GA₃-treated spring and winter barley plants. The higher ear numbers observed in both CCC and GA₃ treated plants were often accompanied by compensatory reductions in grain numbers per ear but these reductions were never so great as to prevent grain yields from being increased by the higher ear numbers. Detailed analyses of the effects of CCC and GA₃ on early growth (tillering, and apical development) were carried out at four different light and temperature (16h/20°C, 16h/15°C, 12h/20°C and 12h /15°C) regimes in the growth cabinet. CCC was seen to induce early and more prolific tillering over a limited period in all four regimes. This observation is discussed in terms of a possible change in the auxinicytokinin ratio in favour of cytokinin. The growth of the main shoot and early primary tillers (T1 and T2) was temporarily retarded and the growth of later shoots enhanced with the result that CCC-troated plants were modified to consist of more uniform shoots with a lass marked hierarchy. Treatment with GA₃ also resulted in an initial rapid rate of tillering which is attributed to an enhancement of the growth of the already growing tiller buds (T1 and T2) at the time of treatment loading to more uniform early shoots. It is suggested that a more uniform plant with less marked size range between its tillers may take up, distribute and utilize its resources (nutrients and photosynthates) more evenly with the result that a greater proportion of tillers survive to produce ears.