The toxicology of a potentially useful antitumor agent, 2,4-diamino-5-adamantyl-6-methylpyrimidine (DAMP), and its ethanesulfonate salt has been studied in beagle dogs after 1 to 20 doses. Two types of toxicity could be discerned: the acute central nervous system toxicity manifested by vomiting, convulsions, and minor hypothermia; and the antiproliferative toxicity, similar to that of other folate antagonists, manifested by diarrhea, anorexia, loss of body weight, and hematological changes as well as changes in blood chemistry. There is evidence of a cumulative effect of the drug with respect to antiproliferative toxicity. Characteristically, the animals could be protected against the antiproliferative toxicity by simultaneous administration of folinic acid. The pharmacokinetics of the ethanesulfonate salt of DAMP was studied after i.v. administration of sublethal doses (5 mg/kg) of tritium-labeled drug. Sixty-three % of the administered dose was excreted in the urine and 10% was excreted in the feces within 48 hr after drug administration. Thus, about 27% of the drug was not recovered, and it is possible that it persists in the tissues for a period of several days. Analysis of the plasma and urine revealed that DAMP was metabolized rapidly. At least 2 metabolites were found in plasma and urine, one lipophilic and one hydrophilic, the latter being the predominant form. Pharmacokinetic data were successfully fitted to a model consisting of central and peripheral DAMP compartments and a DAMP metabolite compartment. DAMP was very rapidly sequestered in the peripheral compartment with a rapid phase half-life of 23 sec. The slower phase of DAMP plasma disappearance had a half-life of 3 hr. The short plasma half-life and rapid metabolism distinguished this drug from other lipophilic antifolates.