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Relationships between some endocrine variables (plasma testosterone and estradiol; in vitro gonadal biosynthesis from pregnenolone precursor and adrenocortical biosynthesis from progesterone precursor; in vitro activity of 17β‐hydroxysteroid de‐hydrogenase (17β‐HSDH) and of aromatizing enzymes in the subcellular fractions of the brain) and isolation‐induced inter‐male aggressiveness were studied. A significant positive correlation was found between fighting behavior, measured by the number of wins in a series of paired encounters, and the activity of brain 17β‐HSDH. A significant inverse correlation was found between fighting behavior and the activity of aromatizing enzymes in the nuclear fraction of the brain. No significant correlations were found between fighting and the other endocrine variables investigated. These results may suggest that the conversions testosterone → androstenedione and testosterone → estradiol in the brain play a role in the control of inter‐male fighting in the mouse. These results may also account for the effect of repeated wins and defeats on brain steroidogenesis.