Nine patients wih Crohn's disease and six healthy individuals were given 400 mg metronidazole twice daily and the concentration was measured in plasma and faeces 6 to 8 h after intake. Quantitative and qualitative bacteriological examinations were performed before and during treatment. The concentrations of metronidazole in plasma varied between 5 and 30 mg/l as measured by liquid chromatography (LC) and between 3 and 20 mg/l as measured by a bioassay (BA) method. There was no significant differences between the results of the two methods in patients but compared with BA the LC values were regularly lower in the healthy subjects. Metronidazole was detected in the faeces of most patients. These findings coincided with a significant reduction of Bacteroides spp in their faeces which was not apparent in healthy individuals in whom metronidazole was not detected in the faeces. Higher faecal metronidazole concentrations were found in patients with active disease and a total colon involvement as compared with the patients with a more quiescent disease and involvement of ileum and ascending colon only.