To study the effect of glucagon immunoneutralization on postoperative changes of urea synthesis, hysterectomized rats were given one injection of a specific high titre antibody against pancreatic glucagon 24 h before operation raising the plasma glucagon binding capacity to values 10-20 times higher than the plasma glucagon concentration in control animals. Earlier studies have shown that the spontaneous rate of urea-N synthesis (UNSR) doubles 3 h after operation, and that the Vmax of the process, the capacity of urea-N synthesis (CUNS) is 50% higher than normal values 24 h after operation. Therefore, the effect of glucagon on UNSR and CUNS were investigated 3 and 24 h postoperatively, respectively. Control animals were given non immune rabbit serum. Glucagon immunoneutralization partly normalized the early increase in UNSR 3 h postoperatively (control: 4.7 +/- 0.3, hysterectomy+serum: 6.7 +/- 0.4, hysterectomy+Gluc-Ab: 5.5 +/- 0.4 mumol (min.100 g BW)-1), but had no effect on the increase of CUNS 24 h postoperatively (control: 7.9 +/- 0.3, hysterectomy+serum: 9.5 +/- 0.3, hysterectomy+Gluc-Ab: 9.8 +/- 0.5 mumol (min.100 g BW)-1). This shows that glucagon is important for the early postoperative increase in the efficacy of urea synthesis, whereas the late increase in capacity seems not to depend on hyperglucagonemia.