We have hypothesized that two of the endogenously synthesized endometrial prostaglandins (PGs), prostaglandin Fzm (PCF,,), and prostaglandin El (PGE1), play a regulatory role in growth control of the rabbit endometrium. PGF,, increases DNA synthesis and PGEl inhibits that effect. Primary cultures of rabbit endometrial cells were used to examine the possible role of these PGs in the mechanism of action of 17P-estradiol on DNA synthesis. Towards this end, binding, second messenger and DNA synthesis experiments were performed. 176-estradiol stimulation resulted in a time dependent (optimal: approximately 6 h) and 170-estradiol concentration dependent (optimal: approximately lo-' M 170-estradiol in phenol red-containing medium) increase in [3H]PGF2, binding. Scatchard type analysis of the binding data revealed an increase in receptor number while the receptor affinity for [3H]PGF2, remained the same as in the control treated cultures. This 170estradiol stimulated increase in PCF2, receptor allowed a suboptimal concentration of PC;Fzu (IO-'M) to increase intracellular levels of inositol polyphosphates, while by itself this concentration of PGF,, caused no significant change in intracellular inositol polyphosphate levels. 17P-estradio1, alone among the several studied steroid hormones, could increase [3H]PCF2, binding. Proliferation studies revealed that, in these primary cultures of rabbit endometrium, 170-estradiol could increase DNA synthesis but not in the presence of indomethacin, unless PGF2, was added to the medium at a concentration (IO-''M) near or above what is normally accumulated in the medium by these cultures. In the absence of 170-estradiol stimulation, addition of these same low concentrations of PGF2, had no effect on DNA synthesis. Apparently, through its effect on the PGF2, receptor, 170-estradiol enhances the PCFz, stimulated DNA synthesis response approximately 100 fold. The DNA synthesis induced by 170-estradiol can be inhibited by PGE?, as can PGFz,-induced DNA synthesis. We propose that 176-estradiol may be mediating its mitogenic effect through an alteration of the prostaglandin agonist:antagonist control of proliferation in rabbit endometrial cultures. In addition we suggest that, if 170-estradiol acts to increase PGF,, receptors as part of its mode of action, this may be of importance in other tissues possessing both prostaglandin and 170-estradiol receptors.