The substance P/neurokinin 1 (SP/NK1) system has been implicated in the processing of negative affect. Its role seems complex and findings from animal studies have not been easily translated to humans. Brain imaging studies on NK1 receptor distribution in humans have revealed an abundance of receptors in cortical, striatal and subcortical areas, including the amygdala. A reduction in NK1 receptors with increasing age has been reported in frontal, temporal, and parietal cortices, as well as in hippocampal areas. Also, a previous study suggests sex differences in cortical and subcortical areas, with women displaying fewer NK1 receptors. The present PET study explored NK1 receptor availability in men (n = 9) and women (n = 9) matched for age varying between 20 and 50 years using the highly specific NK1 receptor antagonist [¹¹C]GR205171 and a reference tissue model with cerebellum as the reference region. Age by sex interactions in the amygdala and the temporal cortex reflected a lower NK1 receptor availability with increasing age in men, but not in women. A general age-related decline in NK1 receptor availability was evident in the frontal, temporal, and occipital cortices, as well as in the brainstem, caudate nucleus, and thalamus. Women had lower NK1 receptor availability in the thalamus. The observed pattern of NK1 receptor distribution in the brain might have functional significance for brain-related disorders showing age- and sex-related differences in prevalence.