We present a direct link between the neutralization of anionic sites by intraperitoneal protamine and a rise in protein passage to the peritoneal cavity during isosmotic peritoneal dialysis in rabbits. Each experiment included two 1-hour exchanges. No drugs were added in the first exchange. In group A (control) there were no drugs in the second hour either. In group B, protamine (50 μg/ml) was added to the second exchange volume. In group C, protamine and heparin (50 U/ml) were added. In groups A and C, appearance curves of metabolites during the first (baseline) and the second (experimental) hours were not statistically different. In group B, differences for urea, glucose and uric acid were not significant, but they were highly so for protein (increase of 100%, p < 0.01). Transperitoneal passage of albumin is substantially enhanced by protamine. Neutralization of protamine with heparin prevents this, showing that availability of anionic sites is the crucial limiting factor. Protamine did not significantly affect the transfer of small neutral molecules.