A cross-over study on metronidazole was carried out with 10 healthy adult volunteers who received tablets (250, 500 and 1,000 mg) and suppositories (500, 1,000 and 2,000 mg). The bioavailability of the suppositories was approximately 90% of the corresponding tablet dose as judged by area under the serum curves and amounts excreted in urine. The absorption was faster after the tablets. The mean curve peaks of unchanged metronidazole after the tablets occurred within 1 h and were 3.7 μg/ml after 250 mg, 9.8 μg/ml after 500 mg, and 11.8 μg/ml after 1,000 mg. The suppositories rendered peaks of 5.5, 9,5 and 14.0 μg/ml 4 h after doses of 500, 1,000 and 2,000 mg, respectively. In the dosage range 250–1,000 mg, the relationship between the dose and the total area under serum curves was similar for tablets and suppositories. Parts of the drug in serum and urine were antibacterially active metabolites. The mean recovery in urine within 36 h was 7–14% of the dose. The serum half-life of unchanged metronidazole (chemical assay) was 8.8–10.5 h. The terminal phase half-life of the total biologically active compound (microbiological assay) was 7