Treatment with methotrexate (MTX) plus 1-beta-D-arabinofuranosylcytosine (ara-C) on Days 1, 4, and 7 after i.p. inoculation of L1210 ascites cells was more effective than treatment with one drug on Days 1, 4, and 7 followed by the second drug on Days 2, 5, and 8. Simultaneous treatment with both drugs was associated with a retention of 1-beta-D-arabinofuranosylcytosine 5'-triphosphate (ara-CTP) but no increase in the activity of deoxycytidine kinase in L1210 cells, whereas pretreatment with MTX 24 hr before the administration of ara-C was associated with approximately 2-fold increases in the level of ara-CTP and of deoxycytidine kinase in L1210 cells. However, from 5 hr after treatment with ara-C, higher levels of ara-CTP were observed in L1210 cells treated simultaneously with both drugs than in cells from animals pretreated with MTX 24 hr before treatment with ara-C. The superiority of simultaneous treatment over sequential treatment and the synergism between MTX and ara-C, previously reported for this schedule of simultaneous treatment, are attributed in part to the MTX-induced retention of ara-CTP and the increased exposure of L1210 cells to ara-CTP that results from the slower clearance of ara-CTP.