Selected fat (FL) or lean (LL) lines of chickens have been further studied. Total lipid and abdominal fat content and size of adipocytes isolated from the gizzard were significantly increased in both sexes of the fat line from 2 to 4 weeks of age onwards. The divergence in abdominal fat content was maximum at 9 weeks of age. Both in the fed and the fasted state, the plasma glucose level was lower in FL than in LL chickens, at hatching and shortly after. This was not, however, associated with higher plasma insulin levels in FL chickens. At 2 weeks of age, insulin content of the pancreas did not differ. From 5-8 weeks of age after ad libitum refeeding or forced-feeding following a fast, plasma glucose increased to similar levels in both lines but in contrast, plasma insulin levels were largely enhanced in FL chickens. At 17 weeks of age, glucose clearance was faster in FL chickens and associated with a slightly (although nonsignificant) higher insulin release. In eggs laid by FL hens, yolk weight was disproportionately increased and albumen glucose content was decreased. During the last third of embryonic development, plasma glucose levels were similar in both lines at the F4 generation and in contrast lower in the FL embryos at the F5 generation. The physiological situation of FL chickens appears therefore very similar to short-lived preobese state observed in mammals.