When paired for 15-min periods for 5-8 consecutive days, castrated, testosterone-treated hamsters consistently assumed the dominant status, based on a higher aggression index (18 +/- 3) and frequency of flank marking (15 +/- 3) as compared to their castrated, untreated subordinate partners (-1.3 +/- 1 and 2.4 +/- 1, respectively). In addition to these hamsters with established dominant/subordinate relationships, control hamsters with no social interactions were killed, and in all animals the vasopressin level in the anterior hypothalamus-medial preoptic area was assessed by counting vasopressin immunoreactive perikarya following immunocytochemistry, or by radioimmunoassay of vasopressin from tissue punches. In the socialized pairs the subordinate hamsters had a significantly (P less than 0.01) lower number of vasopressin staining perikarya in the anterior hypothalamus, specifically the area of the nucleus circularis, than their dominant partners (n = 6 pairs). There was also a significantly (P less than 0.001) lower level of vasopressin immunoreactivity in punches taken from the area of the nucleus circularis in subordinate hamsters as compared to their dominant partners (n = 14 pairs). However, there were no significant differences in the number of perikarya or the concentration of immunoreactive vasopressin between subordinate and dominant hamsters in the supraoptic nucleus, paraventricular nucleus, suprachiasmatic nucleus or bed nucleus of the stria terminalis. The number of perikarya (n = 5 pairs) and concentration of vasopressin (n = 8 pairs) for all vasopressin immunoreactive sites, including the nucleus circularis, were similar for testosterone-treated and untreated hamsters that remained isolated and not subjected to daily aggressive encounters.(ABSTRACT TRUNCATED AT 250 WORDS)