Potential nitrogen mineralization and nitrification were measured in soils from a primary successional sequence developed on sand dunes and from a secondary successional sequence on old fields. Potential nitrogen mineralization in soils from the primary sere increased through the first five stages and then leveled off. Nitrogen mineralization was relatively constant in soils from the secondary sere, except that the highest rates were observed in the oldest site. Nitrification was very closely correlated with nitrogen mineralization in soils from both the primary and secondary seres. Only one site had both substantial nitrogen mineralization and low nitrification. The results of this study do not support the hypothesis that nitrification is progressively inhibited in the course of ecological succession.