The experiments presented in this thesis examined the role of opioids in sexual pleasure, particularly during early formative sexual experiences, on the ability of male rats to associate an olfactory or somatosensory cue with their ejaculation-induced sexual reward state. The results show that opioids are an important substrate of the sexual reward state, and that this state feeds forward to form a conditioned ejaculatory preference (CEP), in which male rats ejaculate preferentially with females bearing or wearing the cue associated with sexual reward. This suggests that sexual reward (a presumably “proximate” cause of behavior) lies at the root of monogamous mate choice in the rat. These data also contribute to a growing body of evidence that the species’ specific sexual strategies of the male rat, often described as promiscuous, evidence a degree of plasticity that can move toward a monogamous mate choice when features of a familiar female (odor or jacket) are associated with sexual pleasure. This conditioned ejaculatory preference for familiar cues suggests that proximate pleasure-related states act as a Pavlovian US that can reinforce cue-related CSs to create partner and mate preferences. Such preferences bring proximate reward states into synch with reproductive choice, potentially enhancing reproductive success. Findings are discussed, as well as further directions.