The present study examined the possibility that the timing of uterine sensitivity for the decidual cell reaction is related to the ability of the uterus to produce prostaglandins, or to respond to prostaglandins with increased endometrial vascular permeability. Immature rats were treated with hormones and ovariectomized so that they were at the equivalent of either Day 4, 5 or 6 of pseudopregnancy. These treatments resulted in rats differentially sensitized for the decidual cell reaction; 5 days after stimulation of the uterus [unilateral intrauterine injection of 50 µl phosphate buffered saline containing gelatin (PBS-G)], the injected uterine horns weighed on average 284, 798 and 303 mg for rats treated on Days 4, 5 and 6, respectively. As indicated by radioactivity levels in the stimulated horn 15 min after the i.v. injection of [¹²⁵I]-labeled bovine serum albumin ([¹²⁵I]-BSA), the increase in endometrial vascular permeability 8 h after stimulation of the uterus was also greatest in animals treated on Day 5. Compared with concentrations in the noninjected uterine horns, the uterine concentrations of prostaglandins E and F, determined by radioimmunoassays, were elevated 15 min and 2 h following intrauterine PBS-G treatment; however, there were no significant differences between days, suggesting that the differential endometrial vascular permeability responses were not due to differences in uterine prostaglandin production. To determine uterine responsiveness to prostaglandins, 10 µg prostaglandin E₂ or its vehicle were injected unilaterally into the uterine lumen of rats treated with indomethacin to inhibit endogenous prostaglandin production. [¹²⁵I]-BSA was used 8 h later to quantify endometrial vascular permeability; the concentration of radioactivity was greatest in the injected horn of rats given prostaglandin E₂ at the equivalent of Day 5 of pseudopregnancy. These data suggest that the timing of the uterine sensitivity for the decidual cell reaction is not related to the ability of the uterus to produce prostaglandins, but may be related to its ability to respond to prostaglandins. Alternatively, the timing may be determined by the production of a permeability increasing mediator which, in addition to prostaglandins, may be essential for changes in endometrial vascular permeability.