The combination of microdialysis and a highly sensitive radioimmunoassay was used in order to monitor the in vivo release of arginine vasopressin (AVP) within hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei of the rat brain. A dialysis probe was inserted into the SON or PVN area and microdialysis was performed in conscious or urethane-anesthetized animals before, during and after hypertonic artificial cerebrospinal fluid (aCSF, with 1 M NaCl) was delivered via the probe. The recovery of AVP in vitro was 1.60%, that of [3H]OH in vitro 14.2% and in vivo 8.44% (SON) and 9.26% (PVN), respectively. AVP was consistently detected in both SON and PVN dialysates; basal levels averaged 0.87 +/- 0.22 pg/30-min dialysate (SON, n = 51) and 0.80 +/- 0.24 pg/30-min dialysate (PVN, n = 6), respectively. Hypertonic aCSF given over a period of 30 min, 60 min or 90 min, resulted in an increased AVP release within the SON which, however, reached its peak (to 8.86-10.27 pg/sample; P less than 0.001 as compared to basal) only in the poststimulation period, i.e. after replacement of hypertonic with isotonic aCSF. An identical osmotic stimulus given 150-210 min after the first one produced similar, though slightly declined, changes in AVP release. In the PVN, AVP release patterns prior to and in response to the first hypertonic pulse were similar to those in the SON; a possible functional difference between the two nuclei is indicated by the lack of a rebound increase in AVP release following the second stimulation. The physiological significance of intranuclearly released AVP remains to be shown.(ABSTRACT TRUNCATED AT 250 WORDS)