Seven- to ten-month-old tilapia hybrids (Oreochromis niloticus×O. aureus), treated for sex inversion early in their ontogeny, were fed daily with pellets containing 17α-ethynyltestosterone (ET) at either 2 or 60 mg/kg for 2 or 11 weeks, respectively. One-g samples of the dorsal musculature were homogenized and extracted with chloroform:methanol (2:1; v/v), washed with water and the organic phase was removed and dried. The extract was dissolved in 80% methanol and the steroid cleaned to some extent on a SEP-PAK C18 cartridge by elution with 3 ml methanol 75%. The isolated fraction was evaporated and aliquots were redissolved in a mixture of acetonitrile (CH3CN) 17% or 46% containing 0.1% trifluoroacetic acid (TFA) in various proportions and analyzed by reversed phase high-performance liquid chromatography (HPLC) at 254 nm. Muscle samples of fish treated for 11 weeks with 60 mg/kg contained detectable amount of the steroid only on the first day after the termination of feeding with ET. Samples taken on the 3rd, 5th and 7th days did not differ from the untreated controls and ET concentrations were below the detectable level of 50 ng/g. The concentrations of the androgen in muscle samples of fish fed with the lower dose of ET were below the level of detection even on the day that feeding was terminated. It may be concluded that the synthetic androgen fed to tilapia even at a high dose, comparable to that used for sex inversion, reaches levels of less than 50 ng/g within 3 days. Residues of the androgen in fish maintained for several months on an androgen-free diet are expected to be negligible. Nevertheless, the possibility of contamination of the holding facilities and the environment with the synthetic androgen remains to be studied.