The hypothalamic slices (containing paraventricular and supraoptic nuclei) of SD rats sectioned with vibratome were incubated in a microchamber and arginine vasopressin (AVP) released from the slices was measured by radioimmunoassay. The effects of corticosterone (B) and bovine serum albumin-conjugated corticosterone (B-BSA) on AVP release were investigated. The results are as follows: (1) B and B-BSA inhibited AVP release within 20 min both in a dose-dependent manner from 10-7-10-4mol/L. At each dose, however, the rapid inhibitory effect of B-BSA was somewhat weaker than that of B. (2) RU486 could partially block the rapid inhibitory effects of both B and B-BSA,although it couldn't by itself change AVP release. But the dose of RU486 blocking the effect of B-BSA was an order of magnitude higher than that blocking the effect of B. (3) With the elevation of Ca2+ in the incubation medium, the rapid inhibitory effects of B and B-BSA were enhanced, while in the absence of Ca2+ their rapid effects attenuated. In these conditions, however, the effect of B-BSA was also weaker than that of B. These results suggested that the rapid inhibitory effect of B-BSA on AVP release from the rat hypothalamic slices was weaker than that of B, and the rapid inhibitory effect of B might be a non-genomic rather than classical genomic one, in which B might change the Ca2+ influx.