Tinidazole was given intravenously to 20 subjects undergoing colorectal surgery in an initial dose of 800 mg, given at induction of anesthesia, and then in doses of 400 mg every 12 h for two days. Series of serum samples and faecal specimens were taken for analysis of tinidazole concentrations. Tissue samples from the gut wall were taken at surgery. The maximum serum concentrations during surgery were reached 0.5-3 h after administration of tinidazole and varied between 13 and 92 microgram/ml. The tinidazole concentrations in the tissue samples were 0.7-18.2 microgram/g. Secondary maxima of the serum concentrations were observed in 16 patients. These occurred 3 to 12 h after the antibiotic administration and ranged between 9.6 and 43.3 microgram/ml. Measureable tinidazole concentrations in the faecal samples were only found in 7 of the 18 patients during surgery and varied between 0.8 and 4.8 microgram/g. Faecal samples were also collected during the investigation period for cultivation of aerobic and anaerobic bacteria. Among the aerobic bacteria, enterococci and streptococci proliferated during the prophylaxis period. Anaerobic bacteria decreased significantly during the same period. After the tinidazole administration period, enterococci and streptococci decreased and the number of anaerobic bacteria increased. No anaerobic strains resistant to tinidazole were isolated. Postoperative infections due to Escherichia coli occurred in six cases.