[3H]PAF binding parameters (affinity constants and binding capacities) were estimated for endometrial membranes obtained on days 3 (when the morulae first enter the gravid uterus), 4, 5 and on day 6, about 22 h before the blastocyst becomes irremovably attached to the endometrium. The two-site receptor system which characterized the binding on a large proportion of purified membranes from days 3 (60%) and 6 (75%) had the following parameters: day 3, Kd1 (nM) 0.15 +/- 0.04, Bmax1 (pmol/mg protein) 0.04 +/- 0.01; Kd2 (nM) 17.60 +/- 6.40, Bmax2 (pmol/mg protein) 2.92 +/- 0.91 (n = 6), and day 6, Kd1 (nM) 0.33 +/- 0.06, Bmax1 (pmol/mg protein) 0.11 +/- 0.02; Kd2 (nM) 7.42 +/- 1.02, Bmax2 (pmol/mg protein) 1.75 +/- 0.31 (n = 3). The remaining membranes, including all preparations of days 4 and 5, exhibited only one class of binding sites which were unlike the putative type 1 PAF receptor. The most significant observations were the apparent pregnancy-associated changes in the parameters of the PAF binding sites. However, regardless of the type of PAF binding exhibited, all the binding sites bound GTP. [3H]PAF dissociation from bound complexes was biphasic at 25 degrees C. The rate constants were k-1, 0.27 +/- 0.09 min-1 and k-2, 3.45 +/- 1.19 x 10(-3) min-1, for the rapid and slow dissociating components, respectively. In view of the inability to resolve the experimental data from days 4 and 5 into multiple sites, and of the occurrence of receptor interconversions on days 3 and 6, it is postulated that there may be cell membrane macromolecular reorganization/remodeling of proteins and cytoskeleton as a result of the arrival of blastocyst in the uterus and/or the dynamic interplay of local uterine PAF and the hormonal milieux during the peri-implantation period of pregnancy.