The transperitoneal kinetics of insulin transfer during peritoneal dialysis and its biologic effects were evaluated in dogs. Changes in plasma glucose and immunoreactive insulin (iRI) during dialysis with 10, 30, and 120 U insulin/ liter of dialysate and during intravenous infusions of insulin at 0.5 or 1.5 U/h, and the recovery of <sup>125</sup>I-insulin added to dialysate were measured. As insulin was increased via the intravenous or the intraperitoneal route, there was a greater fall in plasma glucose and a greater rise in plasma iRI. With intravenous insulin, plasma iRI rose and plateaued within 30 min, with a relatively predictable fall in plasma glucose. With intraperitoneal insulin, plasma iRI rose more slowly, plateaued at 3 h with 10 and 30 insulin/liter dialysate but continued to rise over 5 h with 120 U insulin/liter dialysate. From the observed relationship between dose of intravenous insulin and fall in plasma glucose, the quantities of intraperitoneal insulin absorbed and active were estimated. The percentages of intraperitoneal insulin absorbed, derived from fall in plasma glucose and from loss of <sup>125</sup>I-insulin in dialysate, were 6.1 and 25 ± 5%, respectively. The greater loss of intraperitoneal <sup>125</sup>I-insulin may be explained by its mesenteric binding; subsequent release could account for the slow rise of iRI. Intraperitoneal insulin has been proposed for management of hyperglycemia during peritoneal dialysis of diabetic patients. Present results suggest that intraperitoneal insulin should be used cautiously and in low concentrations because of possible cumulative effects.