Much attention has been given to the use of urea in ruminant feeds since the demonstration by Hart et al. (1939) that non-protein nitrogen (NPN) compounds could be a substitute for protein in ruminant rations. However, the high activity of urease in the rumen decreases the efficiency of urea utilization, because the rate of ammonia release from urea is much faster than the rate of its assimilation by rumen microbes for the synthesis of amino acids (Bloomfield, Garner & Muhrer, 1960). Moreover, if the concentration of ammonia exceeds a certain limit, it causes toxicity to the animals (Froslie, 1977). The efficiency of urea utilization can be increased if the rate of ammonia production is similar to the rate of its assimilation by rumen micro-organisms. This could be achieved either by decreasing the activity of the rumen urease or by modifications of urea in products which result in slow release of ammonia (Helmer & Bartley, 1971). Attempts have been made to increase the efficiency of urea utilization by complexing urea with carbohydrates (Daniels et al. 1971; Muhrer, Harris & Bloomfield, 1968; Shultz, Shultz & Chicco, 1972), sodium bentonite (Britton, Cooling & Klopfenstein, 1978) and carboxy resin (Huston, Scherton & Breuer, 1974). Formaldehyde is known to protect proteins from rumen degradation by its reaction with terminal amino groups (NH2), the e-amino groups of lysine and a number of other side-chain groups of proteins (Barry, 1976; Van Dooren, 1972). Urea contains two amino groups for undergoing chemical reaction with HCHO to give different complexes. Previous work in our laboratory has shown that urea could be complexed with HCHO (Lall et al. 1980; Pal & Negi, 1977). However, in those studies excesses of urea and HCHO were not removed and this produced an artefact in the evaluation of the ureaformaldehyde complexes (UFCs). Moreover, whole rumen fluid was used for in vitro ammonia formation from these complexes. No studies have been made so far in a well-defined system on the biochemical basis of the ammonia-releasing property of the UFCs. We report here the formation and isolation of UFCs, free from urea and HCHO and their hydrolysis with purified rumen urease (EC. 3·5.1.5). Evidence is adduced that UFCs are substrates for rumen urease and can act as slow ammonia-releasing compounds.