Cyclopentenyl cytosine, one of a new series of nucleoside analogues, was found to have marked in vivo antitumor activity in several murine models. It was especially active against an Ara-C resistant L1210 tumor. Inhibition of CTP synthesis was confirmed in mice bearing L1210 ascites. Cyclopentenyl-ara C and cyclopentenyl-8-aza-adenine were found to be cytotoxic to cultured L1210 cells. The pharmacokinetics of thioTEPA administered i.v. or intraventricularly was studied in monkeys and man. TAPA, a metabolite of thioTEPA which is known to be cytotoxic, was observed in all fluids and appeared to have a much slower total body clearance than thioTEPA. A method was developed to quantify dihydrolenperone, a cytotoxic agent with specificity for lung tumors which is currently under evaluation in man.