1. The difference in arginase activity between the tissues of Eisenia and those of Lumbricus shows a relationship to the difference in urea output by living worms of the two species under the same dietary régime.
2. In Eisenia the difference in activity between the tissues of fasting and feeding worms is much smaller than in Lumbricus. The specific outputs of urea by living, fasting and feeding worms likewise differ less than in Lumbricus.
3. These facts strengthen previous evidence in favour of a Krebs-Henseleit type of mechanism for urea production in earthworms.
4. In Eisenia the difference in arginase activity between gut and body wall is similar to, but smaller than, that in Lumbricus, and the body wall makes a major contribution to the total activity.
5. The combined concentrations of ammonia-, amino-, and urea-nitrogen initially present in homogenates of the tissues of these worms are proportional to the combined amounts of the three components excreted per unit weight by living worms of the same species and régime.
6. The two species differ in a number of other properties investigated.