The crude methanol extract of the stem wood of Quassia amara. L. (A-1) significantly caused a reduction in the weight of the testis, epididymis and seminal vesicle, but an increase in that of the anterior pituitary gland. Epididymal sperm counts, serum levels of testosterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were significantly reduced when the rats were treated with the extract. Furthermore the basal and LH-stimulated testosterone secretion of Leydig cells isolated from rats pretreated with the extract was inhibited. These changes seemed to be restored eight weeks after the withdrawal from extract treatments. Two compounds - quassin and 2-methoxycanthin-6-one were isolated after fractionation of A-1. Quassin produced similar biological actions as the crude extract while the effects of 2-methoxycanthin-6-one did not seem to differ from those of the control. Quassin appears to be the antifertility principle of Quassia amara.