That a mammalian embryo might influence the maternal endocrinological and/or immunological systems whilst still in the Fallopian tube has long seemed possible (Fig. 1). Under normal circumstances, the embryo contains a full diploid complement of genetic information and thus the point at issue is just how soon instructions in the genome are expressed for trophic purposes. It could be argued from several points of view that it would be biologically prudent for an embryo to influence the mother whilst still at an early stage of development. Consideration of the extremely intimate apposition of the zona pellucida and the tubal epithelium - perhaps best appreciated in scanning electron micrographs (Fig. 2) - also suggests scope for a mutual exchange of signals. Even so, one of the earliest recorded maternal responses to a developing embryo is the change in permeability of the capillary bed of the uterus that precedes implantation in rodents: this can be demonstrated by the Pontamine blue reaction, a series of bands of dye in the uterine wall corresponding to the location of individual embryos, and is detectable in rats on the fourth day after mating (Psychoyos, 1960, 1973).