Pharmacokinetic characteristics of ornidazole and metronidazole were compared in man. Both drugs were labelled with 14C in the imidazole ring at position 2. Four healthy volunteers were given an oral dose of 750 mg of one of the drugs, followed 2--4 weeks later by the same dose of the other. The same technique for determining the unchanged compound was used for both drugs. Results may be summarized as follows: mean peak concentrations in the plasma of 10.9 mug/ml for ornidazole and of 12.3 mug/ml for metronidazole were reached 2--4 h and 30--60 min after administration, respectively; both drugs showed the same volume of distribution. Their binding to human plasma proteins was less than 15%; the mean half-life of elimination from the plasma was 14.4 h for ornidazole and 8.4 h for metronidazole; the mean radioactivity recovered from the urine and faeces during the first 5 days was 85% of the dose for ornidazole and 91% for metronidazole, the major proportion in each case being excreted in the urine; the radioactivity found in the faeces was 22.1% of the dose for ornidazole and 13.9% for metronidazole.