Beta-endorphin and electroconvulsive shock alter the retrieval of two avoidance tasks when given after the first, but not the second, training session.

1. Rats were submitted to three consecutive sessions, one session per day, of two-way active avoidance or of step-down inhibitory avoidance, and received 1 microgram/kg beta-endorphin intraperitoneally or an electroconvulsive shock immediately after the first or after the second training session. 2. Administration of either treatment after the first session caused a reduction of performance in the second session in both tasks. There was no impairment of performance in the third session. 3. Administration of either treatment after the second session did not affect performance during the third session. 4. Therefore the effect of beta-endorphin and of electroconvulsive shock on active and inhibitory avoidance performance was expressed only when treatments were administered after the first, i.e., novel, training experience. We suggest this effect is on mechanisms acting on retrieval, since the retention performances of all groups for the third session were identical.