We have determined if dietary protein restriction for 1 wk has differential effects on growth, serum IGF-I, and liver growth hormone receptors at various stages of development. Female Wistar rats were fed a low (5%) protein diet for 7 d at 3, 4, 6, 8, and 12 wk of age, whereas controls were maintained on a normal (15%) protein diet. Body wt gain was impaired in the groups fed the low protein diet, despite normal energy intake, and the effect was attenuated with age. Liver cell number (DNA content) was reduced by low protein feeding in the 3-, 4-, and 6-wk age groups (p less than 0.01), but not in the older animals. Protein restriction caused a dramatic decrease in serum IGF-I in the younger animals (90 and 82% reduction versus normal fed age-matched controls, at 3 and 4 wk, respectively; p less than 0.001), and this effect was progressively attenuated with increasing age (49, 40, and 25% reductions of serum IGF-I at 6, 8, and 12 wk, respectively). Changes in serum IGF-I correlated with those of liver cell number (r = 0.80; p less than 0.001). Total and free liver growth hormone receptors were slightly decreased in the low protein diet groups at 4 (p less than 0.05) and 6 wk (total: p less than 0.001; free: p less than 0.01) but not in the other age groups. The occurrence of profound diet induced reductions in IGF-I without proportional reductions in liver GH receptors suggest that the apparent GH resistance occurs at a postreceptor level.(ABSTRACT TRUNCATED AT 250 WORDS)