The pharmacokinetics of metronidazole, its biologically active alcohol metabolite, and its inactive acid metabolite were studied in five noninfected patients undergoing continuous ambulatory peritoneal dialysis and five patients undergoing hemodialysis. The latter were studied on off-dialysis days as a control group. Peritoneal dialysis caused insignificant changes in the apparent volume of distribution, elimination half-life, and total body clearance of metronidazole. Peritoneal dialysis clearance (4.49 +/- 0.88 ml/kg per h [mean +/- standard deviation]) accounted for only 8.9% of total body clearance (50.17 +/- 18.64 ml/kg per h). Analysis of the 24-h area under the serum concentration versus time curves and peritoneal dialysis clearance data for the two metabolites suggested a similar insignificant effect of peritoneal dialysis on their elimination. Metronidazole dialysate concentrations in the first 6-h exchange ranged from 7.6 to 11.7 micrograms/ml. This would suggest that cumulative penetration of metronidazole from the systemic circulation into the peritoneal cavity with dosing every 8 h should lead to adequate concentrations for the treatment of anaerobic peritonitis. For the treatment of systemic anaerobic infections, it would appear at present that metronidazole dosage adjustments are not necessary in patients undergoing continuous ambulatory peritoneal dialysis. The potential for metabolite accumulation was noted in this study. If further studies confirm that excessive serum metabolite concentrations are toxic, dosage reduction in this group of patients may be warranted.