The chick embryo, developing in the egg, is an ideal system in which to investigate the effects of incubation environment on the development of the embryo. We show that raising the temperature of the eggs by just one degree,from 37.5°C to 38.5°C, during embryonic days (ED) 4–7 causes profound changes in development. We demonstrate that embryonic movement is significantly increased in the chicks raised at 38.5°C both during the period in which they are at the higher temperature but also 4 days after their return to the control temperature. Concomitant with this increase in embryonic activity, the embryos raised at higher temperature grow to significantly heavier weights and exhibit significantly longer leg bones (tibia and tarsus)than the controls from ED12 onwards, although mineralization occurs normally. Additionally, the number of leg myonuclei is increased from ED12 in the embryos raised at the higher temperature. This is likely to promote greater leg muscle growth later in development, which may provide postural stability to the chicks posthatch. These changes are similar to those seen when drugs are injected to increase embryonic activity. We therefore believe that the increased embryonic activity provides a mechanism that can explain the increased growth of leg muscle and bone seen when the eggs are incubated for 3 days at higher temperature.