(Lepidoptera: Bombycidae). Tamio INOKUCHI, Masae YAMADA, Kozo NAKAMURA and Hiroshi SHINBO (Sericultural Experiment Station, Ohwashi, Yatabe, Ibaraki 305, Japan). Jap. J. appl. Ent. Zool. 27: 171-175 (1983) After the injection of 14C-urea into mature larvae of the silkworm, radioactivity level in feces, urine, cocoon shell, larval exuvium and body of young pupae was compared between the individuals reared on mulberry leaves and those reared on an artificial diet. In the individuals reared on an artificial diet, 14C-urea was recovered at levels as high as 30% both in the feces and in the urine, 10% in the cocoon shell, 0.5% in the exuvium and 20% in the pupal body. On the other hand, recovery of 14C-urea in the feces and urine in mature larvae fed on mulberry leaves was less than 0.5%, and the radioactivity level in the cocoon shell, pupal body and exuvium was also significantly lower than in those fed on an artificial diet. In the individuals reared on an artificial diet, total recovery of 14C-urea was 99% or more, whereas that in larvae fed on mulberry leaves was as low as 2.5%. Degradation of 14C-urea was determined subsequently by means of capturing of expired 14CO2 at intervals of 30min for 2hr. In the individuals reared on mulberry leaves, cumulative 14CO2 expired in the 4-day-old larvae of the last instar was approximately 5% of the radioactivity injected, whereas that in mature larvae was 9%. Furthermore, 14C-urea injected into the prepupae, pupae and adults was degraded completely to CO2 in 2hr. On the other hand, in the individuals reared on an artificial diet 14C-urea injected into the 4-day-old larvae in the last instar was respired as 14CO2 at the level of 2% in the male and 0.6% in the female, and degradation of 14C-urea was very low in mature larvae, prepupae, pupae and adults, respectively. These results suggest that urease is activated after the spinning period in the larvae fed on mulberry leaves, while in the larvae fed on an artificial diet, activation of this enzyme is not induced.