To determine if dietary protein supplementation in early pregnancy alters total circulating insulin-like growth factor (IGF) levels, genetically similar heifers were fed diets containing different levels of protein in the first and second trimesters of gestation. The groups were: low/low (L/L), fed a diet containing 7% crude protein (CP) per kg/DM (low protein) in the first and second trimesters; high/high (H/H), fed a diet containing 14% CP per kg/DM (high protein) in the first and second trimesters; low/high (L/H), fed low protein in the first trimester and high in the second trimester and vice versa for the high/low (H/L) group. At day 62 of gestation, there was a significant difference (P<0.01) in IGF I concentrations between the high and low protein groups (149 versus 119 ng/ml, S.E. 5.9). There was a strong effect (P<0.001) of protein levels in the second trimester on IGF I levels on days 119, 153, and 183 of gestation but not at day 257. Mean IGF I levels for high and low nutrition in the second trimester were 157 and 97 (S.E. 6.6) for days 119, 191, and 88 (S.E. 12.6) for days 153 and 160, and 67 (S.E. 7.7) for day 183. At day 257, there was a significant interaction (P<0.01) between treatments with the means being 98(ab), 110(b), 116(b) and 79(a gamma) (means followed by a letter in common do not differ significantly, P<0.05) (S.E. 7.5) for H/H, H/L, L/H, and L/L, respectively. There was a significant (P<0.05) effect of protein supplementation in the first trimester on calf IGF I levels at birth with means being 42 and 25 (S.E. 5.2) for high and low protein supplementation, respectively. There was a significant (P<0.01) effect of protein supplementation in second trimester upon IGF II levels and a significant (P<0.05) negative correlation between calf birth weight and IGF II levels.