Domestic chicks will normally peck readily at small colored beads. However, a peck at a bead with an unpleasant taste usually results in a long-lasting aversion to beads of that type. If prior experience ("pretraining") of a bead of the type that is to be used in a later aversive training trial is given to testosterone-treated chicks, it interferes with the retention of avoidance. Interference by pretraining is effective only within sharply defined periods of time: when the pretraining-training interval is either less than 2 min (short term) or greater than about 25 min (long term). When pretraining and training are separated by 90 min or more, the steroid can be effective when injected up to 30 min after pretraining. It appears to change the character of the consolidating memory trace of pretraining in such a way as to make the consolidating memory trace of pretraining in such a way as to make the consolidation of later, contradictory information from training loss likely. The initial storage of this information is apparently unaffected, since retention does not begin to decay until 30-60 min after training. Sort-term interference leads to loss of avoidance within 5 min of training and can be used to demonstrate that the latency of action of the hormone in this task is less than 20 min following a subcutaneous injection.