Insulin sensitivity has been implicated in the variation of fat accumulation at early gestation by as yet unknown mechanisms. In the present study, we have analyzed the insulin sensitivity of lipolysis and lipogenesis in lumbar adipocytes from rats at 0, 7, 14 and 20 days of gestation. In adipocytes of 7-day pregnant rats, we found a 2-fold decrease in both -agonist (isoproterenol and BRL-37344)-stimulated lipolysis and ₃-adrenoceptor protein but not in lipolysis initiated by forskolin or isobutylmethylxanthine, suggesting a modification of the lipolytic pathway at the receptor level. Whereas adipocytes from 7-day pregnant rats showed a 2-fold increase in fatty acid synthesis from glucose, those from 20-day pregnant animals displayed a decreased lipogenic activity. Insulin responsiveness of the lipolytic and lipogenic pathways was analyzed by dose-response experiments, giving evidence for the involvement of improved insulin responsiveness in the enhanced lipogenic and reduced lipolytic activities of adipocytes at early pregnancy. By the contrary, insulin resistance is responsible for lower antilipolytic and lipogenic actions of insulin in late pregnant animals. In conclusion, the present study shows that enhanced adipose tissue insulin responsiveness during early pregnancy contributes to maternal fat accumulation, whereas decreased insulin responsiveness during late gestation modulates fat breakdown.