Constructs containing different segments of the 5' flanking region of the rat growth hormone gene fused to the human growth hormone coding sequences were introduced into fertilized mouse oocytes. As few as 181 base pairs of the rat growth hormone promoter targeted reporter gene expression to the pituitary gland of the resulting transgenic mice. A construct containing only 45 base pairs of the promoter failed to target expression of the reporter to the pituitary, indicating that the pituitary expression is directed by information contained in the segment spanning positions -181 to -45. In the pituitary, immunohistochemistry showed transgene expression mainly in the growth hormone-producing cells (somatotrophs), in a subset of cells producing thyrotropin, and occasionally in prolactin-producing cells. These data establish that cis-active elements contained within the first 180 base pairs of the promoter are sufficient for transcriptional activation of the growth hormone gene in somatotrophs and suggest a functional relationship among growth hormone, prolactin, and thyrotropin gene activation.