Fibroblast growth factor-2 (FGF-2) induces gastrulation of rabbit blastocysts in vitro and is present in the uterine secretion at day 6 after mating. The following study was made in order to show if changes in the uterine FGF-2 concentration or in the FGF receptor concentration of the embryonic tissues point to a regulation of this event. By the use of the ELISA technique and immunohistochemistry, FGF-2 concentration was determined in the endometrial tissue, uterine secretion and blastocyst between day 4 and day 8 of pregnancy, in the uterine secretion after induction of pseudopregnancy, in day 6 blastocysts after in vitro culture, and FGF immunoreactivity was localized in the endometrial tissue. FGF receptor-1 (FGFR-1) concentration was examined correspondingly in the blastocyst. Cross-linking experiments using 125I-FGF-2 were done to identify binding proteins in the blastocyst. In the uterine secretion, FGF-2 was constantly high up to day 6.5 but showed an increase thereafter. Similar values in pseudopregnant uterine secretions indicated that the growth factor was of uterine origin. It was probably synthesized by the uterine epithelium as shown by immunohistochemistry. Under culturing conditions, the blastocyst produced small amounts of FGF-2. In the blastocyst, FGFR-1 as well as binding of 125I-FGF-2 showed a dramatic increase from day 6.0 to day 6.5, coinciding with the onset of gastrulation. Receptor antigenicity was located in the embryonic disc at day 6.5 and day 7.0. Two binding proteins of about 200 and 130 kDa were found by cross-linking. The results indicate that a regulation of growth factor influence on embryonic differentiation is more probable via expression of the embryonic receptor than via differential release of the uterine growth factor.