N-Methylformamide (NMF) has been an agent of considerable interest to oncologists because of its broad spectrum of preclinical antitumor activity, tumor-differentiating abilities, and radiosensitizing and chemosensitizing properties. In this report, the pharmacokinetics of NMF are described, based on data from two phase I studies exploring both iv and oral routes of administration. Mean peak NMF plasma concentrations at recommended phase II doses were 0.46 mmol/L for NMF administered orally, 600 mg/m2 three times/week X 4 weeks every 6 weeks, and 2.78 mmol/L for NMF administered as a weekly iv bolus at 2,000 mg/m2 X 3 weeks every 4 weeks. These NMF concentrations were significantly lower than the concentrations that have been demonstrated to induce antineoplastic and relevant biologic effects in preclinical studies. Plasma disappearance curves were biphasic in the majority of patients; however, 25% of the curves were best fit by a monoexponential kinetic model. Mean alpha half-life and beta half-life values (+/- SE) were 10 +/- 2 and 732 +/- 93 min, respectively. Volumes of distribution for the theoretical central compartment (Vc) and at steady-state (Vss) were 13.8 +/- 1.1 L/m2 and 18.7 +/- 1.1 L/m2, respectively. The mean plasma clearance of NMF was 19.1 +/- 2.1 mL/min per square meter, and the relative contributions to parent compound disposition by respiratory and renal routes were insignificant. No metabolites were identified. Gastrointestinal absorption of oral NMF was rapid and nearly complete; oral bioavailability was calculated to be 0.87. Pharmacodynamic associations were observed between the magnitude of the area under the plasma disappearance curves and hepatotoxicity, the dose-limiting toxic effect of iv NMF, and the symptom complex of nausea, vomiting, and malaise, which precluded dose escalation of oral NMF.