Liver weight of mice was significantly influenced, in order of decreasing prominence, by environmental temperature, infection with Hymenolepis microstoma and sex. Livers of infected and uninfected mice (both sexes) maintained at 5 C for 20 days were proportionally larger (alpha less than or equal to 0.05) than those from corresponding groups of mice kept at 35 C and, except for uninfected males, at 21 C. regardless of temperature, H. microstoma (single-worm infections) produced significant (alpha less than or equal to 0.05) hepatomegaly in all groups of female mice, but, in males, this occurred only in mice maintained at 5 C. At 21 C male mice had proportionally larger livers (alpha less than or equal to 0.05) than corresponding groups of females, but this sexual difference disappeared when mice were maintained at 5 or 35 C. Temperature and sex had a slight but significant effect on bile duct weight. Infection, on the other hand, produced a marked increase in the weight of this organ in both sexes at 5, 21 and 35 C.