Contribution of the pre- and postcommissural fornices to short-term spatial memory was investigated in rats by evaluating the effect of small electrolytic lesions, located stereotaxically, with texts of reinforced alternation in a T-maze. Lesions in the postcommissural fornix were without behavioral effect. Animals with lesions that damaged the precommissural fornix were temporarily impaired in alternation with massed trials. They largely recovered their efficiency by the end of five postoperative sessions. Both the interpolation of irrelevant vestibular input (rotation) and lengthened intertrial intervals (1-4 min) reinstated an alternation deficit that had recovered in the massed-trial condition of testing. It is suggested that the recovery did not represent relearning to alternate but represented experience-dependent switching to parallel neural circuits that also mediate short-term spatial memory.