Nitrification of added N and its inhibition was studied on four Trinidad soils. Nitrification of added N was evident and uniform on an alkaline soil regardless of source, but was more pronounced in the case of three acid soils in the presence of moderate or high dressings of lime. Nitrite accumulated considerably in the presence of lime or where the N source elevated the soil pH. In one fine sandy loam soil (Las Lomas Fine Sandy Loam) in the presence of three sources [urea, ammonium sulphate and sulphur coated urea (SCU)], SCU nitrified best. The nitrification inhibitor 2-amino-4-chloro-6-methylpyrimidine (AM) retarded the production of NO3-N with as low a concentration of 5 p.p.m. AM on an alkaline soil, but its inhibitory effect was only observed on the three acid soils with urea as the source of N. The belief that once nitrification is inhibited within a particular soil zone resumption of the process is quite slow appears to be confined to very acid soils.