Thioguanine (TG) is an antimetabolite with established antileukemic activity. The most pharmacologically rational manner of TG administration is continuous infusion. Intraperitoneal (IP) delivery of TG provides the opportunity to definitively test the concept of high-dose, long-term antimetabolite administration. The high systemic clearance and in vitro activity of TG against ovarian carcinoma suggested that it would be an excellent candidate for IP administration as a prolonged infusion. TG was administered as a 48-hour continuous IP infusion in this phase I/pharmacokinetic study. TG infusions were administered using a portable, programmable pump (Pancretec Provider Model 2000; Pancretec, Inc, San Diego). Twenty-five patients were treated. At a dose of 900 mg/m2/48 h, TG produced unacceptably severe myelosuppression. The dose-limiting toxicity was granulo-cytopenia. Other toxicities were mild: emesis, alopecia, skin rashes, and photosensitivity reactions. IP TG did not produce chemical peritonitis, hepatotoxicity, or mucositis. The pharmacokinetics of IP TG were determined in 16 patients. TG levels were measured by reverse-phase high-performance liquid chromatography (HPLC). At steady state, the mean peritoneal to plasma TG ratio was 1,800 at the maximum tolerated dose (MTD). Steady-state TG levels in the peritoneal cavity and plasma were 2 mmol/L and 1.1 mumol/L, respectively, at a dose of 744 mg/m2. The elimination half-life of TG from the peritoneal cavity was one hour. TG exhibited linear pharmacokinetics over the dosage range investigated. Encouraging clinical activity was seen with IP TG. There was one partial response (PR) and four minor responses (MR). TG can be safely administered by the IP route. The recommended dose for phase II testing is 744 mg/m2/48 h. IP TG has a favorable pharmacokinetic advantage and has demonstrated encouraging clinical activity. Further studies of IP TG infusions are warranted.