Prolactin release from prolactin-cell-containing tissue, the rostral pars distalis (RPD) of the tilapia pituitary, is sensitive to estrogens. This study was undertaken to evaluate the effect of two estrogenic pesticides, p,p'-DDT and o,p'-DDD, on the release of both tilapia prolactins (PRL₁₇₇ and PRL₁₈₈) from the RPD in static incubation, and to examine the effect of exposing tilapia fry to o,p' -DDD on growth and development. We found thatp,p' -DDT stimulated the release of both PRL₁₇₇ and PRL₁₈₈, and that o,p'-DDD inhibited the release of PRL₁₈₈ at the concentrations employed. We also found that 8 months after exposing fry to o,p'-DDD for 28 days, the body weight and liver weight of males and females were significantly different from those of untreated control animals (P<0.01). Gonad weight and pituitary weight were not affected. We also found that 8 months after exposing fry to the estrogen ethynylestradiol (EE₂), the body weight of males was significantly lower (P<0.01) and the body weight of females was significantly higher (P<0.01) than that of the respective untreated controls. Treatment with EE₂ also significantly lowered (P<0.05) the liver weight of males and significantly increased (P<0.01) the gonad and pituitary weights of females compared with their respective controls. The lowest dose of o,p' -DDD used in the in vivo study induced a significant increase in the number of males, an effect opposite to that of EE2.