Anterior pituitaries from female Sprague-Dawley rats (160-200 g b.w) bearing electrolytic lesions of the median eminence were qualitatively and quantitatively investigated by electron microscopy and the findings compared with those from intact animals or sham-operated controls. This study was performed in basal conditions and after stimulation with thyrotropin-releasing hormone. Animals with lesions showed a reduction of both the number of somatotrophic cells and growth hormone granules. After injection of thyrotropin-releasing hormone exocytosis of growth hormone granules from somatotrophs was frequent in rats with lesions of the median eminence but rare in sham-operated animals. The differences were statistically significant.