Urogastrone (UG) increases the rate of mucosal regeneration on patched intestinal defects. Our aim was to determine the effect of the duration of UG administration on regeneration of serosa patched ileal defects in rabbits. Group I (n = 18) were controls. Group II (n = 15), Group III (n = 10) and Group IV (n = 5) received UG 1.5 micrograms/kg/h subcutaneously for 7 days, 14 days or 21 days respectively. Animals were sacrificed at 7 day intervals up to 21 days after patching. Neomucosal growth was significantly greater in the animals receiving UG and was greatest in Group IV. Group II and III animals had less contraction of the patched defect and greater neomucosal surface area than Group I animals at each interval but had a lesser effect than animals receiving UG continuously. Crypt cell production rate was significantly greater in UG treated animals at 7 and 14 days but fell to control levels at 21 days. Prolonging the duration of UG infusion increased the quantity of neomucosa produced by intestinal regeneration. However, UG stimulation of mucosal cell migration and proliferation occurred transiently within 14 days after patching.