The aim of this work was to quantify the brain distribution of the enzyme aromatase in the female baboon with positron emission tomography and the tracer [11C]vorozole using three different quantification methods for estimating the total distribution volume (VT): a graphical method, compartment modeling, and a tissue to plasma ratio. The graphical model and the compartment modeling gave similar estimates to the data and similar values (correlation R 5 .988; p 5 .0001). [11C]Vorozole shows a rapid uptake by the brain followed by a relatively constant accumulation, suggesting the possibility of using the tissue to plasma ratio as an estimate of VT. The highest uptake of [11C]vorozole in the baboon brain was measured in the amygdala, followed by the preoptic area and hypothalamus, basal ganglia, and cortical areas. Pretreatment studies with vorozole or letrozole showed a generalized decrease in brain accumulation and VT. The results suggested that the physiologic changes in gonadal hormone levels accompanying the menstrual cycle had a significant effect on brain aromatase VT.