Ultrastructural changes in male rat neurohypophyses were studied 8 or 30 days after castration, with or without testosterone (T) replacement via capsule implants. Morphometric analyses determined the: (a) amount of neural contact at the basal lamina (BL) of the neurovascular contact zone, (b) average length of individual terminal contacts with the BL, (c) number of neurosecretory terminals per 100 micrograms of BL, and (d) mean number of enclosed axonal processes per pituicyte. Eight days after castration there was decreased neural/BL contact and increased pituicyte enclosure of neurosecretory processes, conditions associated with decreased hormone release. In contrast, T replacement resulted in increased individual nerve terminal length, a measure associated with increased hormone demand. This observation may indicate a stimulatory effect of continuous high-normal circulating levels of testosterone from the capsule implants. There were no differences from control in 30-day castrate rats, but the 30-day rats with T replacement showed morphological indications of increased hormone release. These consisted of increased neural contact with the BL apparently through a significant increase in the number of neurosecretory terminals per unit length of BL. These findings support studies showing a complex feedback interaction between circulating levels of testosterone and vasopressin release.