Pieces of lumbar adipose tissue from 19-day pregnant rats and their virgin controls were incubated in the presence of albumin, glucose, U-14C-glycerol, and either bovine insulin (200 microU/ml) or adrenaline (2.6 microM). The rate of glycerol release in the medium was augmented in both fed and 48-hour fasted pregnant rats. Insulin reduced this parameter in tissues from pregnant rats but not from controls. Adrenaline enhanced it in all groups, expecially in tissues from fed pregnant rats. The rates of CO2, fatty acids and glyceride glycerol formation from glycerol were higher and the effect of insulin was greater in pregnant than in control rats when fed. Fasting produced a decrease in all these parameters, the effect being greater in pregnant than in control rats. The augmented sensitivity of adipose tissue from the mother allows for a rapid switch from the anabolic to the catabolic state according to the necessities of the fetus.