The largest and smallest littermates were chosen by weight from litters of pigs from 38 days' gestation until 1 day post partum. DNA, RNA and two fractions of protein (sarcoplasmic and fibrillar) were measured in the semitendinosus muscle. Total DNA, RNA and protein were found to increase throughout gestation and to have significantly higher values in the larger littermates. DNA concentration reached a maximum at 80-100 days' gestation and then declined. RNA concentration declined throughout gestation while protein concentration fell to a minimum value at 54 days and then increased until term. No difference could be demonstrated between large and small littermates in the concentrations of the above constituents. The RNA/DNA ratio remained relatively constant throughout gestation but was significantly higher in the larger littermates. The protein/DNA ratio increased towards the end of gestation but showed no difference between large and small animals. These results are discussed in terms of the histology of developing muscle and the nutritional status of large and small littermates.