The responses to antifolates of Toxoplasma gondii were investigated by measuring the dihydrofolate reductase (DHFR) activity, quantity of DHFR mRNA, and single-strand conformational polymorphism (SSCP) pattern. Pyrimethamine (PYM) and methotrexate (MTX) were tested as antifolates. When T. gondii was treated with PYM, the viability was decreased by the increasing concentration of PYM, DHFR activity tended to increase as the passage proceeded, and the quantity of mRNA expressed was also increased according to passages. The viability of T. gondii was decreased by the increasing concentration of MTX, but it was maintained over 40% up to 100 ?M MTX. DHFR activity was 77.4% in the 1st passage (1 ?M). 82.2% in the 4th passage (10 ?M), and 141.3% in the 7th passage (100 ?M). But no changes were detected in SSCP pattern of T. gondii exposed to PYM and MTX, both. These results suggested that the response of T. gondii to PYM was regulated by transcriptional level and that, in MTX, the viability of T. gondii was derived from increasing DHFR activity.