A chimeric gene comprising murine whey acidic protein (mWAP) and human growth hormone (hGH) was used to produce transgenic rats that express hGH and secrete it into the blood. Two lines of transgenic rats carrying the mWAP/hGH construct were established: Line 1 was characterized by relatively high levels of serum hGH, and Line 2 had relatively low levels. The secretory profiles of rat GH (rGH) as well as hGH, the transgene product, were obtained in transgenic males and females of Line 2; both hGH and rGH serum levels were flattened with no episodic fluctuations, and the overall mean concentration of rGH was significantly lower than in normal littermates. Although the animals of Line 1 showed an accelerated increase in growth rate, those of Line 2 did not. Nevertheless, the onset of puberty in females, as assessed by vaginal opening and occurrence of first ovulation, advanced by 7-8 days in both Lines of animals. Accordingly, the body weight at puberty of Line 2 transgenic females was much lower than that of normal littermates, indicating that continuous hGH expression could induce precocious puberty without enhancing the growth rate.