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Summary. Uterine vascular permeability and tissue blood volume during the development of the oil-induced decidual cell reaction (DCR) in ovariectomized steroid-treated rats were assessed by measuring the extravascular accumulation of ¹²⁵I-labelled human serum albumin and the tissue content of ⁵¹Cr-labelled red cells 30 min after intravenous administration. Within 15 min of oil instillation into one uterine horn, the vascular permeability of the horn was significantly elevated. Permeability rose to a sharp peak (10 times control levels) 9 h after oil instillation, but dropped to 5 times control values by 12 h and continued a steady decline over the next 7 days. Although a marked increase in uterine weight was associated with the development of the DCR, there was no significant change in blood volume/g tissue until 4 days after oil instillation.

Summary. After suitable sensitization of ovariectomized mice with progesterone and oestradiol, the intrauterine instillation of oil produces a massive decidual cell reaction. Vascular permeability, as reflected by the extra-vascular accumulation of ¹²⁵I-labelled human serum albumin, increased after oil instillation and was maintained at 2–3 times control values for at least the next 3 days. Although oil instillation did not produce a decidual response in females treated with progesterone alone, an increase in vascular permeability (about 2 times control levels) still occurred. This response peaked 8 h after oil instillation and was not maintained. These results indicate that the progesterone-dominated uterus which has not been sensitized with oestradiol cannot be viewed as completely unresponsive to the stimulus of oil and demonstrate that a marked increase in vascular permeability is not itself sufficient to induce decidualization of progesterone-dominated uterine stromal cells. The uterine extravascular accumulation of ¹²⁵I-labelled albumin was increased both in association with tribromoethanol anaesthesia and after oestradiol treatment of progesterone-primed animals. In pregnant mice, the appearance of Pontamine Sky Blue spots provided an earlier indication of implantation than did determination of total uterine extravascular ¹²⁵I-labelled albumin accumulation.