Clinical observations, particularly of the premonitory phase of migraine, suggest the involvement of the hypothalamus in the earliest phases of an attack. Stimulation of the superior sagittal sinus (SSS) in humans produces head pain and permits study of the activated trigeminovascular system in experimental settings. The distribution of neurons expressing the protein product (Fos) of the c-fos immediate early gene was examined in the hypothalamus of anaesthetised (alpha-chloralose) cats. Animals were studied after either 2-h stimulation of the SSS or sham stimulation. Fos protein was detected using immunohistochemistry, and positive neurons were plotted onto standardised templates and counted by a blinded observer. In response to electrical stimulation of the superior sagittal sinus, we found significant activation of the supra-optic nucleus (SON) rising from 3 (0-13) (median, 95% confidence interval) to 53 (31-78; P = 0.005) fos-positive cells. In the posterior hypothalamic area (Hp), fos-positive cells rose from 4 (0-14) to 35 (17-45; P = 0.015) Taken together with other physiological studies, the data are consistent with a role for hypothalamic structures in the modulation of trigeminovascular nociceptive afferent information, and thus for a role in headache.