The effect of phenobarbital (PB) pretreatment on the N-demethylation and antitumor activity of hexamethylmelamine (HMM) was investigated in mice. In mice pretreated with PB, the levels of HMM in plasma, liver, and M5076 tumor were much lower than in mice treated with HMM alone. Plasma area under the curve (AUC) values were 15 +/- 4 versus 37 +/- 28 micrograms/ml X min, liver AUC values were 43 +/- 13 versus 246 +/- 5 micrograms/g X min, and tumor AUC values were 37 +/- 14 versus 109 +/- 24 micrograms/g X min. All of the N-demethylated metabolites except N2N4 dimethylmelamine in plasma, liver, and tumor also reached much lower levels in mice pretreated with PB. The antitumor activity of HMM was significantly antagonized by PB treatment in both M5076 reticular cell sarcoma and PC6 plasmocytoma of the mouse.