There were no differences among study groups in injected dose of [123I]5-IA, the ratio of bolus to infusion, or concentrations of [123I]5-IA activity in the blood. Using multivariate analysis of variance, regional β2*-nAChR availability was compared between smokers and nonsmokers as a function of sex (Table 3, Figure 1, and Figure 2). This analysis demonstrated significant omnibus test results for a main effect of smoking status (F4,102 = 60.24, P < .001) and a sex × smoking status interaction (F4,102 = 2.48, P < .05). Male smokers had significantly higher β2*-nAChR availability than male nonsmokers in the parietal cortex (F1,51 = 9.33, P = .004), frontal cortex (F1,51 = 7.48, P = .009), anterior cingulate cortex (F1,51 = 11.01, P = .002), occipital cortex (F1,51 = 15.09, P < .001), temporal cortex (F1,51 = 9.26, P = .004), cerebellum (F1,51 = 12.03, P = .001), and striatum (F1,51 = 7.96, P = .007), but there were no differences in the thalamus (F1,51 = 1.66, P = .20), consistent with a previous study.22 The β2*-nAChR availability between female smokers and nonsmokers did not differ significantly in the parietal cortex (F1,57 = 0.15, P = .70), frontal cortex (F1,57 = 0.09, P = .77), anterior cingulate cortex (F1,57 = 1.40, P = .24), occipital cortex (F1,57 = 2.75, P = .10), temporal cortex (F1,57 = 0.47, P = .50), cerebellum (F1,57 = 2.98, P = .09), or striatum (F1,57 = 0.64, P = .43), but female nonsmokers paradoxically had higher β2*-nAChR availability than female smokers in the thalamus (F1,57 = 4.37, P = .04) (Table 3, Figure 1, and Figure 2). The β2*-nAChR availability in male smokers did not differ from that in female smokers in any region. Finally, there was a trend toward higher β2*-nAChR availability in female vs male nonsmokers across regions as follows: parietal cortex (F1,55 = 5.74, P = .02), frontal cortex (F1,55 = 5.74, P = .02), anterior cingulate cortex (F1,55 = 3.12, P = .08), occipital cortex (F1,55 = 2.81, P = .10), temporal cortex (F1,55 = 2.91, P = .09), cerebellum (F1,55 = 1.99, P = .16), striatum (F1,55 = 3.08, P = .09), and thalamus (F1,55 = 4.79, P = .03) (Table 3 and Figure 1).