We have investigated the expression of insulin-like growth factor-I (IGF-I) during ontogeny in the chick. IGF-I mRNA was first detectable in whole embryos on day 6, while serum IGF-I could be measured on day 9, the earliest time point examined. Serum IGF-I values rose 10-fold from about 3 ng/ml on day 10 to a prehatch peak of 30-35 ng/ml during days 15-17, and then declined to about 10 ng/ml at the time of hatching. On days 17 and 20 of incubation, IGF-I mRNA was detected in eye, skeletal muscle, and brain, but could not be found in liver or heart until after hatching. During the posthatch period, serum IGF-I rose from 10 ng/ml in the first week to 35-40 ng/ml during weeks 3-6, and liver IGF-I mRNA increased nearly 5-fold from weeks 1-7. The increases observed during weeks 1-3 correlated with a posthatch rise in serum GH from 21 to 37 ng/ml, although GH levels declined over the subsequent 4 weeks without an appreciable change in serum IGF-I values. By contrast, before embryonic day 12 no GH could be detected in the circulation, while IGF-I was 19 ng/ml on day 11. These observations suggest that there is both GH-dependent and GH-independent regulation of IGF-I gene expression in the chick, as has been found in mammals, and support the idea that IGF-I plays a role in chicken embryonic development.