Mineral N concentrations ranged from 133.1 to 167.8 μg g-1 dry soil in fresh casts of the endogeic earthworm Pontoscolex corethrurus fed on an Amazonian Ultisol; this was approximately five times the concentration in non-ingested soil. Most of this N was in the form of NHinf4sup+. N also accumulated in microbial biomass, which increased from a control value of 10.5-11.3 to 67.5-74.1 μg g-1 in fresh casts. During a 16-day incubation, part of the NHinf4sup+-N was nitrified and/or transferred to the microbial biomass. Total labile N (i.e., mineral+biomas N) decreased sharply at first (ca. 50% in the first 12 h), and then more slowly. The exact fate of this N (microbial metabolites, denitrification, or volatilization) is not known. After 16 days, the overall N content of the casts was still 28% higher than that of the control soil. Incubation of the soil before ingestion by the earthworms significantly increased the production of NHinf4sup+in casts. We calculate that in a humid tropical pasture, 50-100 kg mineral N may be produced annually in earthworm casts. Part of this N may be conserved in the compact structure of the cast where the cast is not in close contact with plant roots. © 1992 Springer-Verlag.