In Huntington's disease (HD), corticostriatal and striatopallidal projection neurons preferentially degenerate as a result of mutant huntingtin expression. Pathological deficits in nitric oxide (NO) signaling have also been reported in corticostriatal circuits in HD, however, the impact of age and sex on nitrergic transmission is not well characterized. Thus, we utilized NADPH-diaphorase (NADPH-d) histochemistry and qPCR assays to assess neuronal NO synthase (nNOS) activity/expression in aged male and female Q175 heterozygous mice. Compared to age-matched controls, male Q175 mice exhibited reductions in NADPH-d staining in the motor cortex at 21, but not, 16 months of age. Comparisons across genotypes showed that striatal NADPH-d staining was significantly decreased at both 16 and 21 months of age. Comparisons within sexes in 21 month old mice revealed a decrease in striatal NADPH-d staining in males, but no changes were detected in females. Significant correlations between cortical and striatal NADPH-d staining deficits were also observed in males and females at both ages. To directly assess the role of constitutively active NOS isoforms in these changes, nNOS and endothelial NOS (eNOS) mRNA expression levels were examined in R6/2 (3 month old) and Q175 (11.5 month old) mice using qPCR assays. nNOS transcript expression was decreased in the cortex (40%) and striatum (54%) in R6/2 mice. nNOS mRNA down-regulation in striatum of Q175 animals was more modest (19%), and no changes were detected in cortex. eNOS expression was not changed in the cortex or striatum of Q175 mice. The current findings point to age-dependent deficits in nNOS activity in the HD cortex and striatum which appear first in the striatum and are more pronounced in males. Together, these observations and previous studies indicate that decreases in nitrergic transmission progress with age and are likely to contribute to corticostriatal circuit pathophysiology particularly in male patients with HD.