<p>In the adult mammalian nervous system, sex differences can be manifested independently or in concert with sex-specific hormone cycles, such as the rat estrous cycle. Biological sex and related cycles influence neuronal properties in many brain regions, including the striatum, encompassing the nucleus accumbens (NAc) core, NAc shell, and caudate-putamen (CPu). While neuron soma size and density are commonly assessed in the context of biological sex, these attributes have never been investigated in the striatal regions of adult gonad-intact rodents disaggregated by sex and estrous cycle phase. Thus, we tested the hypothesis that neuron soma size and density would vary by striatal region, sex, and estrous cycle phase. Neuron soma size and density were measured in NAc core, NAc shell, and CPu from adult male rats and female rats in diestrus, proestrus, and estrus phases. Overall, neuron soma size was larger in the CPu than the NAc core and shell. Neuron density was greatest in the NAc shell, followed by the NAc core and CPu. Regarding sex, soma size was larger in male than female NAc shell and did not differ in other regions. Soma density did not sexually differ. Neither soma size nor density differed across estrous cycle phases. These results provide, for the first time, striatal neuron size and density measurements disaggregated by sex and estrous cycle phase and an indication of a sex difference in NAc shell soma size. In contrast, the estrous cycle appears to influence striatal function via other mechanisms than neuronal soma attributes.</p>