Protein turnover in fetal diaphragm, heart, liver, and brain was determined at 21 days gestation in fetal rats whose mothers had received a protein-restricted diet (6% protein) throughout gestation. This diet resulted in severe combined protein-calorie malnutrition. Fetal body weight was significantly decreased at days 19-22 gestation versus controls (27% protein diet) when pregnant animals were protein-restricted (e.g. 40% decrease in body weight at day 22 gestation versus controls). Protein synthesis was determined by intravenous injection of "massive" amounts of [3H]phenylalanine to pregnant animals and measuring free and protein-bound specific radioactivities in fetal tissues. Rate constants for protein degradation were calculated by subtracting fractional growth rate from protein synthesis. Fractional protein synthesis was reduced in diaphragm (0.26 versus 0.41 days-1), heart (0.41 versus 0.52 days-1), and liver (0.35 versus 0.89 days-1) in fetuses from malnourished mothers relative to controls. Similarly, fractional protein degradation was decreased in these fetal tissues-diaphragm (0.03 versus 0.06 days-1), heart (0.14 versus 0.18 days-1), and liver (0.25 versus 0.80 days-1). Reduced protein accretion during maternal malnutrition in these fetal tissues is secondary to decreased protein synthesis out of proportion to the concurrent reduction in protein degradation. Protein synthesis and degradation in fetal brain from malnourished mothers were not altered versus controls. These effects of malnutrition on protein turnover in fetal tissues throughout pregnancy in the rat correspond closely with the effect of protein restriction in young adult rats and the effect of protein-calorie malnutrition on whole body protein turnover in human infants.