Growth and protein accretion were studied in maternal muscle and liver and in foetuses of rats on day 20 of pregnancy. In young rats, weighing 120 g at mating, muscle mass and protein content of three hind-limb muscles, soleus, plantaris and gastrocnemius, increased on average by 7% compared with non-pregnant controls although the rate of muscle protein synthesis was decreased. In mature rats, rates of muscle protein synthesis were also reduced on day 20 of pregnancy but no change in muscle mass was observed. Rates of liver protein synthesis and accretion were increased in the pregnant animals; the effect was larger in the young pregnant rat. Administration of an antibody to rat GH (anti-rGH) for 10 days to young pregnant rats reversed the effect on the same three maternal muscles and resulted in a 9-11% lower muscle mass and protein content, compared with control pregnant animals. In both young and mature dams serum IGF-I concentrations were halved on day 20 of pregnancy, a further small reduction was observed in response to anti-rGH. No significant change in serum insulin or corticosterone levels was observed. Anti-rGH treatment also reduced food intake but foetal weight at 20 days was significantly increased (14%). The effects on maternal muscle were not the result of loss of appetite associated with anti-GH administration as, in rats pair-fed to the intake of the anti-rGH group, maternal muscle and foetal weights were the same as in animals with food available ad libitum. The data suggest that the GH/IGF axis is involved in the partitioning of nutrients between the dam and the foetus.