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Levels of vasopressin (AVP), oxytocin (OXT), and neurophysin (NP) in CSF and plasma of rats were determined during acquisition and retention of passive avoidance behavior. None of the levels of neurohypophyseal peptides in CSF were changed either during the adaptation period, or during acquisition or the retention of this behavior. Moreover, no differences were found in hormone levels in CSF of the various groups of rats subjected to different shock intensities during the acquisition trial. The marked differences in individual latencies of nonavoiding rats, and the differences in latencies due to a different shock intensity applied during the learning trial were not reflected by changes in CSF hormone levels. Neither AVP nor NP levels in plasma were affected by the different shock intensities applied, when measured at 20 min after the learning trial. In contrast, a decrease in plasma OXT levels was observed after application of a shock intensity of 0.25 mA during the learning trial. During retention of the passive avoidance response plasma levels of AVP, OXT and NP were not different from the levels found in the nonshocked groups. It is suggested that under the conditions used in this study the CSF is apparently not involved in the distribution of neurohypophyseal peptides to their possible sites of behavioral action in the brain.Arginine-Svasopressin (AVP) and oxytocin (OXT) both modulate memory processes. AVP affects memory processes by facilitating both storage and retrieval of information (Ader and De Wied, 1972;Rigter, Van Riezen, and De Wied, 1974;De Wied, 1976), whereas OXT generally has an opposite effect. This latter neuropeptide may be a naturally occurring amnesic neuropeptide (Bohus, Kovacs, and De Wied, 1978a). Prom the effects of AVP in rats bearing lesions in various limbic structures as well as from the effects of locally applied vasopressin, it was suggested that AVP may act on intact midbrain limbic circuits, including

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