1. In spring and summer specimens of Porcellio scaber feeding on old poplar leaves excrete from 0.46 to 1.12 ng. dissolved ammonia/mg. body weight x hr. via the faeces, and from 2.3 to 3.25 ng. in gaseous form via the body wall. Non-feeding specimens excrete an average of 11 ng. ammonia/mg. x hr. in gaseous form. In the course of a normal cycle in which periods of feeding activity alternate with periods of inactivity (temporary starvation), faecal ammonia thus constitutes only 10% of the total ammonia released by the animals.
2. If the gaseous component is taken into account, the rate of nitrogen excretion in terrestrial isopods is comparable with that of fresh water and marine species.
3. Oniscus asellus and P. scaber excrete approximately five times more ammonia in spring and summer than in autumn and winter.
4. The faeces produced by P. scaber contain an average of 220 µg. total nitrogen/10 mg. of dry faeces of which 2-4% consist of ammonia.
5. Following abrupt changes of temperature, from 5 to 20° C. and from 20 to 5° C. the rates of the release of ammonia and of the production of faeces follow similar time courses, increasing with an initial Q10 of 3.7 if the change is from the low to the high temperature, decreasing with an initial Q10 of 4.6 if the change is in the opposite direction.
6. It is suggested that in terrestrial isopods nitrogen output in the form of gaseous ammonia more or less balances nitrogen input from leaf litter (the animals' main source of food) and that no reduction of protein metabolism takes place in terrestrial species of isopods as compared with aquatic species.