Experience of an EE or social housing decreases play fighting behavior and social interactions. Many factors could contribute to the modification of emotional or social behavior by rearing environment. Midbrain serotonin and dopamine neurotransmitter systems have been implicated in emotionality. 5-HT neurons located in the raphe nuclei project to many brain areas, including the limbic system, and have been shown to be involved in the modulation of anxiety. On the other hand, the mesocorticolimbic dopaminergic system has been mainly related to reward and motivation. Furthermore, the hypothalamic-pituitary-adrenal axis is an important modulator of stress-related behavior. HPA activity is reflected peripherally by plasma concentrations of corticosterone. In a study of the physiological effects of EE on stress responses, Beltz et al., reported that EE decreased corticosterone concentrations in isolated rats. Although recent studies have led to a better understanding of the effects of rearing environment on emotional and social behavior via neuronal and hormonal regulator systems, the influence of EE rearing on sexual behavior, one of the most important social behaviors, remains unclear. In the present study, we investigated the consequences to male rats of being reared in an EE from early adolescence to puberty on adult sexual behavior, in comparison to rats reared in a SE. Furthermore, to reveal the neurobiological mechanisms underlying the behavioral effects of EE rearing, we focused on the neurotransmitter 5-HT and DA and the hormone corticosterone and testosterone, which have been implicated in the neural regulation of sexual and emotional behavior In th.e present study, we examined the effects of EE rearing on: 1) sexual and emotional behavior, 2) serotonergic and dopaminergic activity following female exposure, and 3) corticosterone and testosterone responses following female exposure. In the present study, rearing in an EE led to notably decreased AB1010 copulatory behavior, a lower number of ejaculations, prolonged ejaculation latencies, longer postejaculatory intervals and other changes. There have only been a few reports of the effects of EE rearing on male copulatory activity. In agreement with the present results, Swanson et al., reported that males reared in an EE showed prolonged latencies to ejaculation compared with control males, and only a few EE males managed to achieve an ejaculation in their study. To date, there is no direct evidence for the neurobiological regulation of changes in copulatory behavior in EE male rats. Although several possible explanations could account for our present results, we hypothesize that the decreased copulatory behavior in EE rats is related to lower emotional responsiveness and central regulation of 5-HT and hormonal responses during mating. First, the nature of our behavioral assessments may explain the emotional responsiveness to copulatory activity. The present behavioral results indicated that EE males decreased their emotional responsiveness in the same open field used in the sexual behavior test.