Whole body autoradiography of 14C-4-testosterone in mice showed the same distribution pattern as earlier investigations by impulse counting on the accumulation of radioactivity in the liver and kidney and the remarkably low concentration in most male sex organs. One major exception was the high concentration of radioactivity found in the ductus deferens. In the testicle the radioactivity was mainly confined to the interstitial parts and in the adrenal a rather high concentration was registered in the cortex. An accumulation of testosterone and/or its metabolites in the corpora lutea of pregnant mice was observed. The findings of testosterone and/or its metabolites in the testicles, adrenals and ovaries are in agreement with the previously demonstrated abilities of these organs to metabolize testosterone.