The inhibition of thymidylate synthase (TS) by the 5-fluorouracil (5FU) metabolite 5-fluorodeoxyuridine monophosphate can be biochemically modulated by leucovorin (LV). LV administration increases the level of reduced folates in tissues, which promotes the inhibition of TS. We have studied the antitumor effect, free 5-fluorodeoxyuridine monophosphate levels, and inhibition of TS in two murine colon tumors at several time points after weekly 5FU or LV and 5FU administration. The antitumor effect of 5FU alone could be potentiated by LV in both tumors. 5-Fluorodeoxyuridine monophosphate levels (212 and 46 pmol/g wet wt. after 2 h for Colon 26 and Colon 38, respectively) were sufficient to mediate TS inhibition, but the levels were not related to antitumor activity. Untreated controls of the 5FU-sensitive tumor Colon 38 had 3 times lower TS levels than did those of the more resistant tumor Colon 26. One course of treatment resulted in a comparable extent and retention of TS inhibition for 5FU and LV/5FU therapy in both tumors. After 1 week there was complete recovery of TS inhibition, but the TS levels in tumors from 5FU-treated mice tended to be higher than the controls, which was more pronounced after three courses of therapy. A 4-fold increase of TS levels was seen in Colon 26 after 5FU therapy. The elevation of TS in this tumor affected the extent of TS inhibition. Tumors treated with 5FU and LV also showed an increase of TS, but to a lesser extent, while the absolute effect on TS inhibition remained the same. This might be related to the potentiating effect of LV on 5FU antitumor activity in vivo in these tumors.