The effect of alaptide, a synthetic derivative of the hypothalamic MIF, upon social memory of intact adult males (Experiment 1), castrated males (Experiment 2), ovariectomized females (Experiment 3) and aged females (Experiment 4) in rats was studied. Social investigation oriented toward a juvenile male (22-24 days old) was used as the measure of social recognition. In addition, sniffing behaviour by adult animals was evaluated during the exposure. Alaptide at the dose of 1.0 mg/kg, and 0.5 mg (Experiment 1), was injected sc immediately after the initial exposure. Reexposure of animals to the same or a novel juvenile was performed 120 min later. Time spent in social investigation by alaptide-treated intact males as well as by ovariectomized females was significantly reduced upon reexposure to the same juvenile. This reflects memory specific effect of alaptide, i.e., the animal is able to recognize individual olfactory characteristics of the juvenile. On the contrary, alaptide was ineffective in enhancing social recognition in castrated males and in aged females. Because the alaptide-treated animals spent less time in social investigation if reexposed both to the same and to a novel juvenile, the finding is considered as reflecting a non-specific effect. The results are discussed from a point of view of the importance of androgens or estrogens in the modulation of social recognition and of a male's or female's capacity to process and to retain olfactory information under the influence of alaptide. With the exception of intact adult males, all tested animals devoted less time to sniffing behaviour during reexposure so that habituation to the experimental environment took place in them. Thus, social investigatory behaviour and sniffing behaviour seem to be two different behavioural systems with different neurobiological mechanisms.