Non-breeding stickleback males were gonadectomized or sham-operated in winter. Treatment with long photoperiod at high temperature stimulated sexual maturation in control fish. Brains were freeze-sectioned and punches were taken with hollow needles from different diencephalic regions and the punches incubated with [19-3H]-androstenedione. After extraction of steroids with CH2Cl2 tritium activity remaining in the water phase was measured. This gives a measure of aromatization as tritiated water and formic acid is formed when [19-3H]-androstenedione is aromatized to estrogens. Aromatase activity in the area containing the nucleus preopticus and the nucleus anterioris periventricularis was dramatically higher in sham-operated males than in gonadectomized ones or in the initial controls. There was no effect of gonadectomy in the other studied regions, including the nucleus lateralis tuberis, lobi inferioris, nucleus posterioris periventricularis and thalamic regions.