In an attempt to develop an animal model to study the etiology of brain dysfunction in relation to early life experience, we tested the hypothesis that early-life stress during specific postnatal time windows affects long-term potentiation (LTP) reinforcement in adolescence. Male Wistar rat pups were stressed by separation from their dams for 24 h at postnatal day (PND) 4, 9, or 18. The animals were tested for reinforcement of LTP at adolescence (9 weeks old) by exposing them to a 2-min swim-stress. Here, we show that maternal separation during (at PND9) but not at the beginning (at PND4) or after (at PND18) the stress-hyporesponsive-period of the hypothalamic-pituitary-adrenal-axis impairs emotional LTP-reinforcement in adolescent animals. Thus, this in vivo model allows the investigation of physiological and pathophysiological emotional information processing at the cellular level in freely behaving adolescent animals.