Abstract A single injection of CXM (experimental groups) or NaCl (control groups) was associated with a pretreatment (PT) which consisted of free exploration of a Y-maze. Then, during a learning session the animals were given avoidance conditioning in the same apparatus, either 1 day or 3 weeks after the PT. The injection of CXM impaired the acquisition of conditioning only when it took place 1 day after the PT. In another experiment the injections were associated with an aversive PT (uncontrolable shocks delivered in the maze). Under these conditions, the subsequent acquisition of conditioning was impaired in the control subjects for both delays PT-learning. The performance of the experimental animals was: (1) similar to that of the controls when 1 day elapsed between the PT and the learning session; (2) better than that of the controls when the interval between the two sessions was 3 weeks. The results obtained after CXM injection were interpreted in terms of short-term aversive effects and true amnesia at longer intervals.