Abstract Monopterus albus is a sex-reversing hermaphrodite that changes from functional female, through an intersexual stage, to the functional male phase during its life cycle. Since previous findings indicated that steroid hormones may play some part in the process of sex reversal of Monopterus, an investigation on the role of sex steroid hormones was made using hormone administration together with biopsy study. Various androgens, including methyltestosterone, testosterone and 11-ketotestosterone, at different dosages failed to bring about precocious sex reversal in the female and no significant changes in the ovaries were observable after treatment. Methyltestosterone also failed to enhance the development of testicular lobules in the intersex animals. On the other hand, oestrogens injected into the intersex and male fish caused suppression of spermatogenesis and destruction of testicular lobules, but failed to cause the fish to change in the female direction. By the use of cyanoketone, an inhibitor of steroidogenesis, and the administration of methyltestosterone, it was demonstrated that androgens failed to bring about sex reversal even in the absence of endogenous oestrogens. Hence the failure of androgens to stimulate the development of male germ cells in the female phase is probably not due to any inhibitory effects by endogenous oestrogens. It is therefore concluded that sex steroids do not appear to play any primary role in the triggering of natural sex reversal of this protogynous fish.