The study was performed to evaluate the ability of phosphatidylcholine (PPC) to modulate the toxicity of 5-fluorouracil/folinic acid (5-FU/FA) in rats. Twenty-eight female Wistar rats were divided into four series, each consisting of 6--8 animals and administered 0.5 ml isotonic solution i.v. once a day. The series were as follows: 1) saline; 2) 20 mg 5-FU + 4 mg FA; 3) 30 mg 5-FU + 6 mg FA; 4) 2 weeks daily orally 50 mg PPC/kg body weight (b.w.), on day 12, 30 mg 5-FU + 6 mg FA. As an indicator of membrane luminal impairment, 72 h after an i.v. administration, peptidase activities were measured by using an in situ perfusion model. The i.v. application of 5-FU/FA causes a clear reduction of enzyme activities in comparison to the control group. In investigations with rats, we have shown that a low concentration of oral PPC (50 mg/kg b.w.) cannot protect the luminal mucosa membrane from being injured by the enterotoxic action of 5-FU/FA.