Steriod hormones regulate differentiation and proliferation of several cell types. They interact with intracellular receptors that belong to the family of ligandinducible transcriptional enhancer factors, In contrast to steroids, peptide growth factors interact with membrane receptors, producing intracellular signals that are responsible for short-term responses. Steroids also produce extragenomic, rapid responses similar to those evoked by the peptide growth factors. Within a few seconds, vitamin D3 treatment activates phospholipase C from intact enterocyte~.~ Within seconds, progesterone stimulates calcium influx in sperm.5 Within minutes, progesterone directly causes an increase of oxytocin receptor binding in the postenor ventromedial hypothalamus.6 TABLE 1 presents some of the nongenomic effects of steroid hormones. We observed that estradiol complexed with its receptor stimulates in uitro a 67-kDa tyrosine k i n a ~ eT. ~h e kinase has been isolated and some of the in uitro substrates have been identified. The enzyme phosphorylates synthesized human wild-type (HEGO) and a point-mutated (HEO) estradiol receptor (ER).'.' Phosphorylation of the tyrosyl residue at position 537 of the hormone binding domain of the synthetic ER confers hormone binding with high affinity to the receptor."' The physiological relevance of this observation has been proved recently by finding the same tyrosyl residue phosphorylation in whole MCF-7 cell ER." FIGURE1 shows a model of the 67-kDa tyrosine kinase activation by the receptor occupied by the hormone. This model, extensively verified in uitro in our laboratory, immediately suggests that estradiol directly stimulates tyrosine kinase(s) and protein tyrosine phosphorylation in uiuo. Stimulation of protein phosphorylation on tyrosine is a well-known early effect of peptide growth fact o r ~ ~ ~an-d" i s also involved in cell transformation induced by retroviral oncogene~. '~L, 'i'k e these growth factors, estradiol may stimulate tyrosine phosphorylation of intact cells. This possibility has been investigated using human breast cancer MCF-7 cells under conditions of estradiol-stimulated growth.