Correlations between the specific binding of radiolabelled adrenoligands in frontal cortex and hypothalamus, and plasma testosterone levels were investigated in inbred mouse strains. Binding values were structure and genotype dependent. There were significant negative intrastrain and interstrain correlations between hypothalamic binding of 3H-clonidine, an imidazoline with alpha 2-adrenoceptor agonist specificity, and resting testosterone levels. Neither binding of the alpha 1-adrenoceptor antagonist 3H-prazosin nor of the beta-adrenoceptor antagonist 3H-dihydroalprenolol (DHA) correlated significantly with hormone concentrations. The inverse correlation between hypothalamic 3H-clonidine-binding and plasma testosterone levels suggests a possible role of alpha 2-adrenoceptors in the negative regulation of testosterone plasma levels.