The expression and hormonal regulation of chimaeric genes containing the 5'-flanking regions of the normal human growth hormone (hGH-1), the variant hGH (hGH-2) and chorionic somatomammotropin (hCS-1) genes fused to the chloramphenicol acetyl transferase (CAT) gene has been examined after transient transfection into cultured rat pituitary (GC), and non-pituitary (HeLa and Rat 2) tumor cells. As assessed by levels of CAT activity, the hGH-1 and hCS-1 gene hybrids were expressed at 5- to 25-fold higher levels in GC cells than in HeLa or Rat 2 cells. The hGH-2 gene hybrid was expressed at very low levels in all 3 cell types. Triiodothyronine treatment of transiently transfected GC cells had little effect on CAT activity from the hGH-1 gene hybrid but increased CAT activity from the hCS-1 gene hybrid. A slight but significant increase in CAT expression was detected with both genes after dexamethasone treatment. The data indicate that elements present on the hGH-1 and hCS-1 genes' 5'-flanking DNA are required for the efficient expression of these genes in GC cells.