The nutrient partitioning of growing pigs was altered through nutrition and castration in order to investigate the relationship between serum insulin-like growth factor 1 (IGFl) and protein deposition and serum cholesterol and fat deposition. In a 2 × 2 factorial experiment 18 entire and 18 castrated male pigs, of 20 kg initial live weight, were given either 2·25 (low) or 3·4 (high) times maintenance energy requirements to a scale based on live weight. Nitrogen and energy balances were measured over 7-day periods when the pigs reached about 30, 60 and 90 kg. Fasting blood samples were taken at each weight and serum was analysed for IGFl and insulin and total- and high density lipoprotein (HDL)-cholesterol. Protein and fat deposition, IGFl and cholesterol concentrations were higher for pigs given the high, as opposed to the low, feeding level. Protein deposition and IGFl concentrations were higher for entire males, as opposed to castrated males, whereas fat deposition and cholesterol concentrations were higher in castrated than in entire males. IGFl and protein deposition increased with age for entire but not for castrated males. Conversely, fat deposition increased with age in castrated but not in entire males. There was no effect of age on serum cholesterol. Serum IGFl was correlated with protein deposition at 30, 60 and 90 kg (r = 0·40, r = 0·63 and r = 0·67; P < 0·05, P < 0·002 and P < 0001 respectively, no. = 36). Serum total cholesterol and low density lipoprotein (LDL)-cholesterol were correlated with fat deposition at 60 and 90 kg (r = 0·65 and r = 0·54; both P < 0·001 for total cholesterol; r = 0·66 and r = 0·50; both P < 0·001 for LDL-cholesterol). Insulin levels were similar for pigs in all treatment groups. It is concluded that serum IGFl and cholesterol may give a useful indication of protein and fat deposition in pigs of between 60 and 90 kg live weight.