The pharmacological properties of a new type of anti-cancer agent, 1-(2-tetrahydrofuryl)-5-fluorouracil (FT) and uracil (U), in a molar ratio of 1:4, were investigated and compared with those of FT, 5-fluorouracil (5-FU) and uracil. UFT at doses larger than 874.8 (FT: 270.0) mg/kg p.o. produced slight central nervous depression in mice and rats, as demonstrated by prolongation of the thiopental anesthesia, analgesic activity in the acetic acid stretching test, an anticonvulsant effect on maximal electroshock convulsions and decrease in body temperature. In rabbits UFT at a dose of 291.6 (FT: 90.0) mg/kg p.o. produced an increase in the drowsy EEG pattern. UFT at high dose produced clonic convulsions in mice and rats and cardiac fibrillation in rabbits. UFT had slight cardiorespiratory effects in anesthetized rabbits and dogs and at a dose of 291.6 (FT: 90.0) mg/kg i.v. it increased the voltage of the T-wave in ECG in anesthetized dogs. At high doses UFT inhibited gastrointestinal transit in mice and it caused emesis in dogs. At doses larger than 291.6 (FT: 90.0) mg/kg p.o. it had a diuretic effect and at higher doses it slightly increased the blood glucose level in rats. FT at equal doses to UFT had the same effects as UFT and the effect of 5-FU was similar to that of UFT. The pharmacological activity of uracil was much less potent. It was concluded that the pharmacological effect of UFT was almost entirely due to FT, however, UFT had much less effect than FT in inducing clonic convulsions, increasing the voltage of the T-waves in ECG and inducing emesis, its depressant effect is probably due to its constituent uracil.