1-(2-Chloroethyl)-3-isobutyl-3-(beta-maltosyl)-1-nitrosourea (TA-077) is a new masked antitumor agent, which is hydrolyzed by maltase to give rise to a more cell-permeable and, hence, more cytotoxic metabolite, TA-G. TA-077 was intravenously administered to mice, rats, guinea pigs, rabbits and dogs and the time-courses of blood concentrations of TA-077 and TA-G were followed. The time-course patterns of blood TA-077 in these animals were markedly different from one another depending on the level of plasma maltase, while those of blood TA-G were all similar except in the early stage after the injection; i.e., the blood concentration of TA-G reached a peak shortly after the injection and decreased rapidly with a half-life of 6 to 15 min, and the higher the level of plasma maltase, the earlier the time of peak appearance. Tissue concentrations of TA-077 and TA-G in major organs were also measured at various times after intravenous injection of TA-077 to guinea pigs and VX-2 tumor-bearing rabbits. In both species, tissue concentrations of TA-077 did not exceed the blood concentration and rapidly decreased to insignificant levels by 30 to 45 min. The tissue level of TA-G in the kidney, a maltase-rich organ, was always the highest among the organs examined in both species. The concentration of TA-G in the VX-2 tumor was relatively high. Possible significance of tissue maltase to the cellular uptake and antitumor effect of TA-077 is also discussed.