Growth hormone (GH) gene expression in mammals is regulated by the interaction of the transcription factor Pit-1 with two binding sites within the proximal promoter. Four sequences, homologous to the mammalian Pit-1 motif occur in the rainbow trout (Oncorhynchus mykiss) GHII (rtGH) gene promoter, two of which partly overlap. The three regions containing these putative Pit-1 binding sequences were protected from deoxyribonuclease I digestion by nuclear extracts of GC cells, a rat pituitary tumor cell line producing Pit-1. In gel shift assays, nuclear proteins from GC cells and from trout pituitaries were found to interact specifically with one of these protected sites. Transfection experiments showed that the rtGH promoter is transcriptionally active in GC cells, the response being strongly enhanced in the presence of a cAMP analogue. The results demonstrate that rat Pit-1 binds to and activates the rtGH promoter, indicating that the basic mechanisms regulating GH gene transcription have been conserved between fish and mammals.