Auto-oxidation products of plant phenolics in alkaline medium, such as gallic acid and pyrogallol were used to show antigonadotropic activity. The complex mixture of oxidation products was extracted from aqueous medium successively by ethyl ether and ethyl acetate. The fractions obtained were tested on a model of mouse Leydig cells in vitro. All compounds used inhibited luteinizing hormone-stimulated testosterone secretion during 6 and 24 h culture whereas basal secretion was stimulated by pyrogallol oxidation products. Not only low molecular weight substances extracted by organic solvents but also the remaining water soluble, dark brown, high molecular weight products were found to be antigonadotropically active.