The effects of restricting protein intake on plasma insulin were studied in pregnant pigs, fetuses and the developing offspring. Pregnant pigs were fed diets containing 18%, 3% or 0.5% protein throughout the gestation period. At 10, 13 and 15 weeks of gestation, fetuses were removed from the uterus after bleeding the dam. Plasma samples were used for insulin determination by a radioimmunoassay procedure. At week 15 of gestation, plasma insulin levels were significantly higher in pregnant pigs fed 18% protein and also in their fetuses than in the other two groups. There was a high correlation between fetal insulin level and fetal growth rate (r = 0.84). Two-day-pld pigs from another set of pregnant pigs fed the diet containing 18%, 3%, or 0.5% protein during gestation were cross-fostered to control nursing dams and weaned at 4 weeks of age to a standard diet. Plasma smaples obtained at regular intervals were used for insulin determination. Offspring of pigs fed 0.5% protein during gestation had consistently low insulin levels in postnatal life in spite of cross-fostering and standard feeding after weaning. It appears that one of the stimuli which control maternal insulin secretion and placenta transfer is the maternal protein intake while severe maternal protein restriction might contribute to the low levels of insulin in the progeny during postnatal life.