SUMMARYDuring flower development in tobacco (Nicotiana alataLink&Otto) changes in peroxidase activity in the unpollinated style and in the corolla were determined both qualitatively and quantitatively. In both the peroxidase activity increases during maturation, especially towards the end of the growth period. Immediately after anthesis the peroxidase activity in the style decreases during two days, followed by a strong increase. The changes in peroxidase activity in the style are different from those in the corolla.The alterations of isoenzyme patterns during flower development are primarily quantitative rather than qualitative. Only one new isoenzyme appears during the growth in style and corolla.The possible relationship between the variations in peroxidase activity in the style and the physiological barrier preventing self‐fertilization is discusse