The actions and biological responses of anti-estrogens are a function of: the experimental conditions, the parameters, the organ and the animal species considered. Target tissues for estrogens in the guinea-pig during the perinatal period are interesting models to explore the action of anti-estrogens. The summary of the data indicates: (1) In the fetal uterus of guinea-pig in in vivoexperiments (after injection to the maternal compartment) tamoxifen acts as a real agonist concerning growth, as a partial agonist concerning the stimulation of the progesterone receptor. (2) In in vitro experiments (in organ culture of fetal uterus or in isolated cells) anti-estrogens (tamoxifen or 4-hydroxy-tamoxifen) act as antagonists and also inhibit the effects provoked by estrogens. (3) In the uterus and vagina of newborn guinea-pigs, tamoxifen and its derivatives: 4-hydroxytamoxifen and N-desmethyltamoxifen act as real agonists concerning the uterotrophic and vaginotrophic effects, and also stimulate the amount of DNA per organ, but concerning the progesterone receptor in the uterus, in the short treatment anti-estrogens act as partial agonists but they have no effect in the long treatment. In the vagina in the short treatment anti-estrogens provoke no significant effects, but in the long treatment they are full agonists. In neither of the two biological responses studied (growth and progesterone receptor) does tamoxifen and its derivatives block the action of estradiol. (4) The use of a monoclonal antibody to the estrogen receptor revealed quantitative differences in the activation of the estrogen receptor when bound to estradiol or tamoxifen. This observation was in agreement with the lesser extent of binding to DNA-cellulose of the tamoxifen-estrogen receptor complex as compared with the estradiol-estrogen receptor complex. This fact suggests an impaired activation of the estrogen receptor induced by tamoxifen which might be related to the different biological responses provoked by estrogens and anti-estrogens.