It is well known that biological constituents, including enzymes, biogenic amines and their metabolites, do not remain at a constant level, but vary in a rhythmic manner. One of the most studied rhythms is the daily one in relation to light and darkness. A bimodal diurnal pattern was, for instance, reported for the concentration of dopamine and its principal metabolite, homovanillic acid (HVA) in the dopaminergic structures of the rat brain (Piepho & Friedman, 1968). We now report the fluctuations of HVA levels in the striatum during a rhythm other than the 24-h one, i.e. the sexual cycle. Sexually mature female Charles—River rats, weighing 200±10 g, were kept in Makrolon cages (six/cage), at room temperature (22 °C), with relative humidity 60%, controlled artificial light for 12 h (06.30–18.30 h) and in restful conditions, with food and water ad libitum. Vaginal smears were examined during three consecutive oestrous cycles. Rats with