Twenty Nigerian soil samples, mainly from field experiments at the International Institute of Tropical Agriculture (IITA), were used in a study of the effects of clearing and cropping on the C, N and S reserves of soils under secondary lowland rain forest. Soils that were cropped for 2 yr usually contained less total C and N than the corresponding forest soils. This decline was less rapid when crop residues (maize) were returned to the soil as a mulch than when they were removed. The soils were incubated in the laboratory either fresh, or after exposure to stress treatments (air-drying or fumigation). The amounts of N mineralized by the fresh untreated soils were small and not clearly related to the cropping history. Both air-drying and fumigation caused a marked increase in the mineralization of N and the amount released was related to the cropping history. Cropping caused an even greater decrease in these ''stress-labile'' N reserves than in total soil N, so that cropping causes a decline in both the quantity and quality of soil organic matter. Again, the decline was less where crops were grown under mulches. A term percent stress-labile N (PSN), defined as the percentage of the total N mineralized when fumigated soil is incubated under defined conditions, is proposed as a measure of the quality of soil organic N. Under secondary forest, the PSN ranged from 3.4-4.2%; on cultivation it declined, in one case to less than 1%. The amount of C in the soil biomass was calculated from the size of the flush of decomposition caused by fumigation. Biomass C usually decreased on cultivation, the decrease being less under mulches. There was a close linear relationship (r = 0.94) between the flush of N caused by fumigation and the amount of biomass C in a soil and a rough measure of biomass C can be obtained by multiplying the N flush by 8.