Phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) regulation of the GnRH gene was studied in two mouse GnRH neuronal cell lines, Gn11 and NLT. TPA treatment of NLT cells grown in low serum conditions did not alter endogenous mouse GnRH mRNA levels, indicating that the endogenous mouse gene is not regulated by phorbol esters under these conditions. This result is confirmed in transfection studies in which TPA treatment did not change expression of a mouse GnRH-luciferase reporter gene construct. In contrast, TPA treatment stimulated expression of a human GnRH-luciferase reporter construct, correlating with the expression of the protooncogenes c-fos and c- jun. TPA stimulation of the human GnRH gene is mediated by a consensus AP-1 site located at -402 to -396 bp, TGACTCA, which specifically binds c-fos and c-jun in Gn11 and NLT cells and recombinant c-jun in gel mobility shift studies. In contrast, the rodent GnRH genes, when aligned for maximum homology, contain a DNA sequence with a 1-bp difference, TGTCTCA from the human gene. In gel mobility shift studies, this DNA sequence does not form a complex with Gn11 or NLT nuclear extract or with recombinant c-jun. This is the first demonstration of species-specific differences in phorbol ester regulation of GnRH gene transcription and could, in part, explain differences in reproductive function among mammals.