The hypoxic radiosensitizers misonidazole or demethylmisonidazole were administered i.p. in a 2-liter volume to 6 patients affected by advanced ovarian carcinoma, and the pharmacokinetic course of the two drugs was studied. The clearance of misonidazole and demethylmisonidazole from the peritoneal fluid was 19.1 and 12.4 ml/min, respectively. At 3 hr after drug administration, both radiosensitizers had peritoneal fluid concentrations more than 8 times larger than in the plasma. The concentration x time exposure in the peritoneal fluid was 3.2 times larger than in plasma for misonidazole and 7.6 times for demethylmisonidazole. The advantage of i.p. delivery compared with systemic delivery decreases with distance from the peritoneal surface, but the advantage may be maintained for up to 1 mm or 100 cell layers. These differences between the two routes of administration provide a rational basis for the expectation that a substantial increase of the therapeutic benefits of misonidazole and demethylmisonidazole in potentiating radiation therapy or chemotherapy can be expected in treating tumors confined to the i.p. space.