Leptin, a recently identified hormone, is believed to reduce appetite and maintain body weight. The mRNA of leptin is expressed only in mature adipose cells. To clarify the regulation of leptin gene expression in adipocytes, 3T3-L1 adipocytes were treated for 16 h with various agents known to modulate lipid metabolism, and then the leptin mRNA was measured by the reverse transcription-polymerase chain reaction method. Interestingly, both norepinephrine and isoproterenol reduced the level of leptin mRNA to about 20% of that found in untreated control cells in a dose-dependent fashion. The maximum reduction occurred at 100 nmol/l of either norepinephrine or isoproterenol, and the half-maximal effect was observed at approximately 3 nmol/l norepinephrine and approximately 1 nmol/l isoproterenol. Propranolol reversed about 50% of the reduction by either norepinephrine or isoproterenol. In contrast, phentolamine did not inhibit the reduction by either norepinephrine or isoproterenol. Moreover, both cholera toxin and dibutyryl cAMP decreased the level of leptin mRNA to about 10% of that in control cells in a dose-dependent fashion. The maximum effect was elicited at 100 ng/ml cholera toxin and 100 micromol/l dibutyryl cAMP. The concentration producing the half-maximal effect was approximately 1 ng/ml cholera toxin and approximately 50 micromol/l dibutyryl cAMP. Dibutyryl cGMP, however, did not affect leptin gene expression. These results suggest that a signaling pathway that results in the activation of protein kinase A regulates leptin gene expression in 3T3-L1 adipocytes.