We have investigated whether insulin or growth hormone (GH) can affect growth of transplanted fetal rat paws by direct and/or indirect actions. Paws from 15-day rat fetuses were transplanted under the kidney capsule of adult female hosts, and grown for 7 days. In hypophysectomized (HX) or diabetic hosts their growth was reduced by 65% and 35%, respectively. The direct effects of insulin and GH were studied by inserting a catheter connected to an osmotic minipump into the left renal artery. Thus transplants on the left (infused) kidney were exposed directly to much higher concentrations of hormone than were those on the right (uninfused) side. In HX hosts, infusion of GH (0.1 microgram X g body wt-1 X day-1) had no effect on growth of paws on either kidney. At 0.3 or 1.0 microgram X g body wt-1 X day-1 GH caused a dose-related restoration of transplant growth that was equivalent on both sides. In diabetic hosts infusion of insulin (2 U X kg body wt-1 X day-1) partially restored growth of paws on the infused kidney without affecting those on the right. Insulin at 6 U X kg body wt-1 X day-1 caused full restoration of growth of the paws on the left and a partial increase in those on the right. At 9 U X kg body wt-1 X day-1 insulin caused full restoration of transplant growth on both kidneys. These results indicate that insulin promotes normal growth of transplanted fetal paws by direct and possibly indirect mechanisms, but the effects of GH are apparently only indirect.