The pharmacokinetics of metronidazole, a drug effective in vitro against most anaerobic bacteria and promising in treating anaerobic infections, are described. Serum and urine levels after single and multiple doses in 10 adult male volunteers were measured by an agar well diffusion bioassay using clostridial species as the test organisms under anaerobic conditions. Peak serum levels averaged 11.5 mug/ml and 6.2 mug/ml after single 500-mg and 250-mg doses, respectively. Renal clearance was only 10.2 ml/min per 1.73 m(2), and less than 20% of the administered dose was recovered in the urine as active drug in 24 h. The average serum half-life was 8.7 h, and there was no protein binding as determined by an ultrafiltration method. With multiple doses of metronidazole (500 mg four times a day and 250 mg three times a day), blood levels increased progressively for the first few doses and then leveled off, with no significant accumulation occurring between 3 and 7 days. On 250 mg three times a day, serum levels just before the 8 a.m. dose (12 h after the preceding dose) on the third day averaged 3.9 mug/ml, and before the 8 p.m. dose, 5.7 mug/ml. For the higher, 500-mg dose (four times a day) regimen, the corresponding minimum serum levels were 13.1 mug/ml at 8 a.m. and 21.3 mug/ml at 8 p.m. Peak levels would have been about 10 mug/ml higher, and since the minimum inhibitory concentrations of most anaerobes including Bacteroides fragilis are less than 6 mug/ml, these concentrations should be highly effective therapeutically, even for severe infections.