Spontaneous alternation behavior in a T-maze was studied in mice of the BALB/c strain implanted with a stimulation electrode in the lateral hypothalamus (LH). For animals receiving no electrical stimulation spontaneous alternation rates decreased as a function of the increasing time interval between the first and the second trial (intertrial interval ITI). Thus, as compared to chance rate alternation (41.4%), mice significantly alternated at the 30 sec ITI (73.2%), exhibited a slight but not significant tendency to alternate at 30 min (51.8%) and performed very close to chance at 6 hr (39.3%). Stimulation of the LH, using a low current intensity (5 microA) during the first arm-choice, modifies subsequent arm-choice tested on the second trial. The direction of the effect, however, is highly dependent on the ITI. Thus, as compared to nonstimulated animals, stimulated mice show a strong preference to return to the previously visited arm when the ITI was 30 min, while for an ITI of 6 hr they, in fact, significantly avoid that arm (alternation). These results indicate that LH stimulation induces facilitative effects on memory for events occurring on the 1st trial as clearly shown for the 6 hr ITI. In addition to its general facilitative effect, stimulation could induce a place-reward association of short-duration which could explain the fact that animals tend to return to the same arm at 30 min. Alternatively, it is suggested that stimulation might have short-lasting amnesiant properties. Both of these hypotheses are discussed.