Cells obtained from turtle forebrain can be maintained in culture for at least 3 weeks. The cells are capable of aromatizing [3H]androstenedione and [3H]testosterone to estrone and estradiol and several C-19 metabolites. There are marked differences in the quality and quantity of the products formed from the two substrates. Conditions in living cells favor accumulation of 17-hydroxylated steroids. Aromatase activity as measured by estrogen yield increases with time in culture. Estrogen content of 14-day-old cultures may be enhanced or reduced by addition of natural or synthetic steroids. This system may provide a model for studying the regulation of brain aromatization, an essential step in the expression of androgen action on certain behavioral and neuroendocrine responses.