1. Heavy-breed (HB) chicks differed from light-breed (LB) ones in their propensity to be overfed. Whereas in the LB chicks the amount by which they could be overfed reached 70 % more than the food consumed daily by the ad lib.-fed chicks, in the HB chicks the maximal excess was only 13 %.
2. Overfeeding caused a slight but statistically significant increase in the linear growth rate (shank length) of the LB chicks, with an opposite effect in the HB chicks.
3. Overfeeding increased the weight of the crop, proventriculus, small intestine, pancreas, liver and adipose tissue but had no such effect on the heart, cerebrum or cerebellum.
4. Overfeeding had no effect on the specific activities of the pancreatic digestive enzymes, liver xanthine dehydrogenase, or tryptophan oxygenase (EC 22.214.171.124). The increase in the total activities was due entirely to organ hypertrophy.
5. Obesity induced in young chicks had no residual effects on the adult LB chicks, but reduced the linear growth of the adult HB chicks.
6. An explanation for the difference between breeds in response to overfeeding at an early age is discussed.