From stigma to basal end, styles of Nicoliana alata have a decreasing gradient of total peroxidase activity. As shown by starch gel electrophoresis this gradient is accompanied by differences in the contribution of particular peroxidase isoenzymes. Under the influence of pollination and subsequent pollen tube growth the activities of several peroxidase isoenzymes show changes which differ in the various stylar parts. The increase in activity of certain peroxidase isoenzymes is transmitted like a wave from the stigma to the basal end of the style. For some isoenzymes the distance covered by this activation wave depends on the pollen tube length, whereas for one of the peroxidases it only seems to depend on the time passed since pollination. The increase of the peroxidase isoenzymes which was previously suggested to be necessary for continuation of pollen tube growth through the style appears to be a consequence of pollen tube growth. The changes observed in the activity of one of the peroxidase isoenzymes (10) are extensively discussed. Preliminary evidence is provided which suggests that this isoenzyme is involved in the rejection of incompatible growing pollen tubes.