One dose of 500 mg metronidazole (M) was given to 21 patients before appendectomy and the concentrations of the parent compound and the major metabolite hydroxymetronidazole (OH-M), assayed in serum and tissue homogenates of appendix and subcutaneous tissues by high pressure liquid chromatography. Based on determinations in one specimen per patient during the initial 1.25-4 h after end of the 50 min infusion, the levels of M in relation to serum were 60.8% (+/- S.D. 19.8) for the appendix and 29.0% +/- 8.7 for subcutaneous tissue. The corresponding values for OH-M were 103.9 +/- 28.2 and 57.0% +/- 16.8. The slope of the log-linear regression curves of M corresponded to elimination half-life (T 1/2) values of 5.2 h in serum, 2.7 h in appendix and 2.6 h in subcutis tissue, but the differences were not statistically significant. The values of OH-M were similar for serum and appendix. OH-M had T 1/2 of 7.9 and 6.1 h for serum and subcutaneous tissue and 21.3 h in the appendix, but the values were not significantly different.