The lag in insulin release in response to glucose is an obstacle to the development of hybrid pancreatic devices, in which an artificial membrane protects transplanted islets against immune rejection. We designed a U-shaped bioartificial pancreas, in which the blood channel surrounds the islet chamber consisting of two flat membranes; blood circulates successively above the first membrane and then in the reverse direction, below the second membrane. Isolated rat islets were introduced into the chamber, which was perfused with Krebs buffer, and the kinetics of insulin release in response to glucose was determined. During a 20-min, 2.8-20-mM, square-wave glucose stimulation, insulin release in the effluent of the device rose from 0.7 +/- 0.2 to 3.2 +/- 1.0 ng/100 islets/min (P less than 0.05) within 3 min, and reached a maximal level of 12.8 +/- 3.3 ng/100 islets/min at 10 min; 5 min after the return of the glucose concentration to substimulatory level, insulin release dropped from 11.3 +/- 1.5 to 8.0 +/- 1.7 ng/100 islets/min (P less than 0.05), and reached basal value (1.0 +/- 0.2 ng/100 islets/min) 40 min after the end of the stimulation. A 0.1-mM/L/min ramp increase in glucose concentration triggered a significant rise in insulin release (P less than 0.02) when the glucose concentration reached 5.3 +/- 0.2 mM; islets concomitantly perifused within a chamber set up without membrane responded to the same glucose stimulation 5 min earlier. For up to 1000 islets, insulin release in response to glucose was linearly correlated to the number of islets (N = 12, P less than 0.01), indicating that insulin did not significantly inhibit its own secretion in this system. Finally, during glucose stimulation, the insulin concentration in the effluent from the chamber was found to be four times the concentration present at the turning point of the blood channel, suggesting that insulin was transferred into the perfusing medium in part by a countercurrent flux of ultrafiltrate crossing the membranes. We present herein the kinetic modelling of glucose and insulin transfer in this "ultrafiltration chamber," whose functional characteristics are compatible with closed-loop insulin delivery.