Studies were undertaken to investigate the effects of nitrapyrin (2-chloro-6-(trichloromethyl) pyridine), dicyandiamide (DCD), soil moisture and temperature on the transformation of labelled ammonium-N; and to look at the of nitrapyrin and shade on the efficiency and distribution of labelled N fertilizers, applied to maize grown in soil columns and subjected to a temperature and moisture regime similar to that prevalent in the humid tropics. Nitrapyrin was superior to DCD at the soil moisture and temperature regimes investigated. Nitrapyrin maintained 80 and 70% of the applied 15NH4+ - N after 6 weeks of incubation at 20 and 30°C, respectively, but nitrification was rapid at 40°C. In the presence of DCD, nitrification of ammonium-N proceeded rapidly at all the soil moistures and temperatures studied. The poor nitrification inhibition exhibited by both chemicals between 30 and 40°C may render them ineffective in the humid tropics. Loss of N by leaching occurred mostly during the first 4 weeks of growth. The N loss under reduced radiation (102.6 kg N ha-1) was more than double that under full sunlight and was mostly native soil N. Leaching loss of labelled N was < 0.1 % from planting to maturity. Incorporation of nitrapyrin in urea and ammonium sulphate did not significantly improve N recovery by the maize plants, but did slightly increase the amount of N lmmbolized in the soil. Sodium nitrate was absorbed as efficiently by the plant as ammonium sulphate was, and both were absorbed significantly better than urea. Total fertilizer recovery at harvest in plant and soil was ? 85 % for sodium nitrate and ammonium sulphate and 75% for urea.