Abstract 1. 1. The effect produced by gonadectomy on dopamine (DA) and noradrenaline (NA) levels in the diencephalon and the rest of the brain of male and female rats during postnatal development has been studied. 2. 2. DA and NA metabolism or biosynthesis seems to be regulated by the ovarian hormones, directly or by means of hypophysary hormones, since both catecholamine levels rise acutely during postnatal development when ovariectomy is performed. In contrast with controls, the NA level is not stable at 45 days, but continues rising to day 60. 3. 3. Orchidectomy also acutely increases the level of diencephalic DA, but in contrast with females, its concentration progressively decreases, being at day 60 the same as in the controls. The reasons that cause this normalization, in the absence of testicular androgens, are unknown. 4. 4. In the same way, the extirpation of the testicles increases the diencephalic concentration of NA, the concentration change is similar to the control one: however, the level is higher. There is also a clear difference from the gonadectomized females, in which the diencephalic NA rises during 45–60 days. 5. 5. Gonadectomy does not significantly alter the level of DA or NA in the rest of the brain.