Despite intense investigation, the modulation of LHRH release, essential to reproduction, is not fully defined. In this study we investigated whether dynamic transformations of individual LHRH terminals in the median eminence of the hypothalamus occurred as a function of gonadectomy in rats, using immunocytochemistry and electron microscopy with quantitative image analysis. One day after castration, the distance between LHRH terminals and the basal lamina was reduced by 50% as LH levels rose significantly. By contrast, in females, this distance did not decrease until 6 days after ovariectomy, coincident with a delayed rise in LH levels. The percent area of each immunopositive terminal occupied by LHRH reaction product was smaller in intact males than females and increased after castration to reach a maximum 3 weeks after castration. By contrast, in females, the greatest percent area was observed in control diestrous females and decreased to a minimum 3 weeks after ovariectomy. Three weeks after gonadectomy, distance and area measurements no longer displayed significant sex differences. Transformations of LHRH terminals may be modulated by direct action on LHRH terminals or intervening neuronal or nonneuronal elements in the median eminence. Modulating factors may derive from local elements or circulating factors bound to local extracellular matrix.