Variations in phosphatidate phosphohydrolase (PAP) activity and triacylglycerol concentration were measured in pregnant and hormone-treated non-pregnant female rats. PAP activity in adipose tissue was elevated by 61% during pregnancy. The increase in the enzyme activity was paralleled with a rise in serum triacylglycerol concentration (44%). Estradiol injecting into non-pregnant rats increased PAP activity of both adipose tissue (19.8%) and the liver (26%). Progesterone also elevated the enzyme activities of adipose tissue and liver by 10% and 55%, respectively. Both hormones increased serum triacylglycerol concentration (20-29%). The data demonstrated that hypertriglyceridemia observed during pregnancy was mediated through the hormonal effects on PAP activity, a key enzyme in glycerolipid metabolism.