Abstract: Transgenic common carp (Cyprinus carpio) have been produced by microinjecting a construct pMThGH, a human growth hormone (hGH) gene with a mouse metallothionein-1 (MT) gene promoter in pBR322, into fertilized eggs. The transgenic fish and the descendent generation F1 and F2 have exhibited growth enhancement. After hypophysectomy, the growth of the pMThGH-transgenic F2 common carp continued while the growth of control fish, including non-transgenic common carp and crucian carp (Carassius auratus), ceased. This cessation in crucian carp could be overcome by subcutaneous injection of recombinant hGH. It has therefore been demonstrated that hGH, both from the transgene expression in vivo and from biosynthetic source via subcutaneous injection, can replace the carp's endogenous growth hormone to stimulate the growth rate following hypophysectomy.