The specificity of steroid hormone-induced sex determination was investigated in the red-eared slider, Trachemys scripta, a turtle with temperature-dependent sex determination. All eggs were incubated at either a female-producing temperature (31°C) or a male-producing temperature (26°C) and received control or experimental treatments at stage 17-18 of embryonic development. A variety of treatments induced female sex determination at the male producing temperature. Oestradiol-17β, diethylstilboestrol (DES) (an oestrogen agonist) and norethindrone (NET) (a progestin with reputed oestrogenic as well as anti-oestrogenic properties) were the most effective in inducing female sex determination. Other reputed oestrogen antagonists/partial agonists (i.e. tamoxifen, nafoxidine and clomiphene citrate) were also capable of inducing female sex determination, but to a lesser extent. A high dosage of testosterone resulted in the production of some females (7 of 15 hatchlings) whereas dihydrotestosterone had no detectable effect on sex determination. This latter finding suggests that testosterone could be acting via aromatization to oestradiol-17β. A few females resulted from eggs that had been treated with aromatase inhibitor, 1,4,6-androstatrien-3,17-dione (ATD) (3 of 97), the antiandrogen hydroxyflutamide (1 of 55) and progesterone (3 of 36), suggesting the possibilities of non-specific effects of these compounds when used in large dosages. Alternatively, metabolites of these compounds may be oestrogenic. Collectively, the results at the male-producing temperature are consistent with the hypothesis that steroid-induced female sex determination is mediated via an oestrogen-specific receptor. With the exception of a single male turtle (produced from an egg treated with 4-androsten-4-ol-3,17-dione (4-OHA)), the various treatments were not able to induce male sex determination at a female-producing temperature. This suggests that either (i) sex determination has little or no sensitivity to steroid hormones at a female-producing temperature, (ii) the effects of steroid hormones are overridden by the effect of the female-producing temperature or (iii) the effects of steroid hormones are similar to the effects of the female-producing temperature and are thus masked. The effects of he treatments on Mullerian duct differentiation and development were also recorded. NET exerted distinct and equivalent effects at both male- and female-producing temperatures, with many of the females exhibiting a hypertrophied cranial portion of the Mullerian ducts but lacking the caudal portion (i.e. Mullerian duct agenesia). A similar effect was shown for the majority of females produced from eggs treated with DES and for one turtle produced from an egg treated with oestradiol-17β at the female-producing temperature. Certain dosages of oestradiol-17β and oestradiol-3-benzoate resulted in females with full length hypertrophied Mullerian ducts. Tamoxifen, nafoxidine and clomiphene citrate treatments did not stimulate Mullerian duct agenesia, but they did appear to act as oestrogen agonists in blocking Mullerian duct regression in some males. Further, although clomiphene citrate did not stimulate agenesia, it did produce hypertrophic Mullerian ducts in some males and females. The latter result is consistent with the hypothesis that different receptors could be involved in the differentiation versus hypertrophy of the Mullerian ducts.