The mechanisms through which gonadal steroids exert feedback actions upon the activity of the GnRH neurons are not understood. Using a series of GnRH-LacZ transgenic mice we have examined the manner in which gonadal steroids suppress GnRH mRNA expression in male and female mice. The long-term gonadectomy of 5.5-GNZ-3.5 transgenic mice resulted in significant increases in cellular GnRH mRNA expression (P < 0.05) and plasma LH concentrations (P < 0.01) in both sexes. However, cellular levels of LacZ mRNA and -galactosidase, which provide an index of GnRH gene transcription, were only elevated in males following gonadecotmy. This sexually differentiated response was also observed in mice gonadectomized for 2 weeks. Estrogen replacement in gonadectomized males returned transgene expression to intact levels. Experiments in transgenic mice with 3' and 5' deleted GnRH-LacZ constructs revealed that the suppressive influence of estrogen upon LacZ transcription in the male required a critical element located between -5.2 and -1.7kb of the GnRH promoter. These studies show that the suppression of GnRH mRNA expression by estrogen in the male involves a decrease in GnRH gene transcription that is dependent upon a distal GnRH promoter element. The same mechanism does not exist in females indicating that gonadal steroids suppress GnRH mRNA levels in a sexually dimorphic manner.