The equilibrium between lactone and salt forms of camptothecin (CPT) and its derivatives including 9-nitrocamptothecin (9-NC) depends on pH, binding to albumin and other factors. Their antitumor activity is associated with the lactone form. Our goal was the development of dosing regimens optimal for chemotherapeutic activity of the drug. The effect of p.o., i.v. and i.m. administration on the tumor uptake of [3H]-CPT or [3H]-9-NC in tumor-bearing nude mice was studied by whole-body autoradiography. In all cases, [3H]-CPT or [3H]-9-NC accumulated mainly in the gastrointestinal tract. Comparatively lower levels of drug were detected in liver, kidney, tumor, and other sites. Consistently high tumor/blood ratios following oral administration of drug suggest this route as the most effective way of treatment. Within 4 h of i.s. administration of 2 mg/kg CPT or 1 mg/kg 9-NC to mice, lactone forms were 57-81% and 47-95% of total plasma drug levels, respectively. However in plasma of humans treated p.o. with varying doses of CPT or 9-NC, lactone forms were only a minor component of total drug levels. It is concluded that ratios of lactone/total drug are much higher in mice than in humans, which influence the therapeutic efficacies these drugs in the two species.