Insulin and tumor necrosis factor alpha (TNF alpha) produce potent and opposing physiological signals in adipocytes. However, genes that are co-regulated by the hormone and cytokine during and after adipocyte differentiation have not been characterized. Using 3T3-L1 cells, we have studied the regulation of the expression of genes encoding acyl-CoA synthetase (ACS), and stearoyl CoA desaturase-1 (SCD-1), two enzymes that play key roles in the metabolism of long chain fatty acids. Insulin is required for triggering the transcriptional activation of the ACS and SCD-1 genes at an early stage in adipocyte differentiation. In mature adipocytes insulin elicits a 4-fold increase in the rates of transcription of the two genes. However, when 3T3-L1 adipocytes are treated with TNF alpha the cytokine causes a 75-90% decrease in the levels of ACS and SCD-1 mRNAs. The decline in mRNA content is associated with similar decrements in the rates of transcription of the ACS and SCD-1 genes. Thus, the ACS and SCD-1 genes are subject to stimulation and counter-regulation (at the transcriptional level) by insulin and TNF alpha, respectively. The opposing effects of insulin and TNF alpha are observed in developing and terminally differentiated adipocytes. Unlike the ACS and SCD-1 genes, the genes that encode the lipogenic enzymes lipoprotein lipase and malic enzyme are not subject to counter-regulation by insulin and TNF alpha at the transcriptional level in 3T3-L1 adipocytes. These observations on the control of ACS and SCD-1 expression suggest possible mechanisms by which adipocytes can markedly adjust their capacity for long chain fatty acid metabolism in response to external stimuli.