In order to monitor changes in the apical cell membrane of rabbit uterine epithelium which are postulated to be a precondition for trophoblast attachment, the marker enzymes: alkaline phosphatase, aminopeptidase M, gamma-glutamyl transferase and dipeptidyl peptidase IV were investigated during the periimplantation phase. Endometrium of early pregnancy (implantation chamber, interblastocyst endometrium; 5-8 days post coitum, d p.c.) was compared with specimens obtained at hCG-induced pseudopregnancy (p. hCG) to distinguish between membrane changes regulated by maternal plasma steroid hormones and such which might be induced locally by blastocyst-derived signals. All enzymes tested showed their main activity at 5 d p.c./p. hCG. The weakest reaction in this series of stages was generally found at 8 d p.c. (interblastocyst segments) or at 8 d p. hCG. In contrast to the rest of the epithelium, the implantation chamber retained high activity of dipeptidyl peptidase IV, and the activity of alkaline phosphatase even raised here again at 7 and 8 d p.c. indicating a direct local influence of the blastocyst on the luminal epithelium. The results suggest that 1) considerable changes occur in the composition of the apical plasma membrane of the uterine epithelium when the endometrium enters the "receptive state", 2) the overall trend is towards a loss of apical-type characteristics of this membrane domain and 3) the changes are modulated both systemically (by plasma steroid hormone levels) and locally by signals from the implanting blastocyst.