The sexually dimorphic area (SDA) of the gerbil hypothalamus is essential for mating in male gerbils. To determine if it affects mating through its connections with the ventral part of the lateral septal nucleus (LSv), the caudal part of the medial bed nucleus of the stria terminalis (caudal BSTm), or the medial amygdala-amygdalohippocampal area (MA-AHi), these connections were severed. Unilateral cell-body lesions were made in the SDA and in the contralateral LSv, caudal BSTm, or MA-AHi. Controls received the same lesions ipsilaterally. Other gerbils received lesions in the caudal BSTm and contra- or ipsilateral MA-AHi. Only contralateral lesions of the SDA and caudal BSTm severely impaired mating. Because contralateral lesions of the SDA and MA-AHi, or BSTm and MA-AHi, did not mimic this effect, the BSTm neurons that are needed for male sexual behavior through their connections with the SDA do not simply relay information to or from the MA-AHi.