Studies are described that sought the basis for a discrepancy in values for a key kinetic parameter of methotrexate transport (influx Vmax) in L1210 cells derived alternately from biochemical or pharmacokinetic measurements. Our results show that, within a short period of time following administration of a therapeutic dose of methotrexate to leukemic mice, influx of this folate analogue measured in L1210 cells removed from these mice was markedly stimulated. Enhancement of [3H]methotrexate influx in these cells was observed within 15 min of drug administration, was maximum (up to 3-fold) within 2 to 3 h, then decreased with time until 24 h when influx was at the control level. Measurements of [3H]methotrexate influx in cells removed from drug-treated mice were made after a period of incubation in drug-free medium to allow for efflux of exchangeable drug. Enhanced influx of [3H]methotrexate was accounted for by an increase in influx Vmax (influx Km was unchanged) and was further enhanced (to a total of 5-fold) by coadministration of leucovorin. Also, enhancement of influx of [3H]methotrexate in L1210 cells did not occur following administration of 1-beta-D-arabinofuranosylcytidine at a therapeutically equivalent dose to leukemic mice or following exposure of these cells to methotrexate or methotrexate with leucovorin during growth in culture. Methotrexate therapy did not affect all transport systems, since the same therapy of leukemic mice had no effect on influx of the purine nucleoside analogue, 9-beta-D-arabinofuranosyl-2-fluoroadenine, in these same L1210 cells. These findings suggest that stimulation of [3H]methotrexate influx in L1210 cells during therapy with this folate analogue was not due to transstimulation during exchange between folate compounds and was not related to the antiproliferative effect of methotrexate on these tumor cells. The coadministration of cycloheximide with methotrexate to leukemic mice at a dose which markedly inhibited 3H-leucine incorporation into L1210 cell protein severely diminished the stimulation of [3H]methotrexate influx. However, in L1210 cells removed from leukemic mice treated with methotrexate, there was no increase compared to control cells in affinity labeling with the N-hydroxysuccinimide ester of [3H]methotrexate. This suggested that the effect of cycloheximide was not on increased synthesis of folate transporter and that increased rate of translocation of folate transporter, rather than increased amount of transporter, accounted for the increase in [3H]methotrexate influx.(ABSTRACT TRUNCATED AT 400 WORDS)