Serum concentrations of tinidazole and metronidazole have been measured in healthy female volunteers who received a single dose of 2 g. of each drug in a cross-over study. Bioassays against T. vaginalis showed that metronidazole achieved higher peak concentrations (mean 81 mug./ml.) than did tinidazole (mean 67 mug./ml.), while assays for unchanged drug showed higher peak concentrations of tinidazole )mean 51 mug./ml;) than of metronidazole (mean 40 mug./ml9). This discrepancy is probably due to the presence in the serum of active metabolites of metronidazole. The longer half-life of tinidazole led to significantly higher serum concentrations (by bioassay and chemical assay) of tinidazole than of metronidazole from 6 hrs onwards.