In vitro chimaeras have been produced by injecting [3H]thymidine-labelled 8th day embryonic ectoderm, derived from the anterior, distal or posterior regions of the egg cylinder, into unlabelled synchronous embryos. Injected embryos were cultured for 36 h and the distribution of donor cells was analysed autoradiographically. One series of orthotopic injections was carried out and the results indicate that the developmental fate of embryonic ectoderm in the posterior part of the embryo is to form mesoderm, both embryonic and extraembryonic. Heterotopic injections of distal and posterior embryonic ectoderm demonstrate that these tissues readily conform to the colonisation patterns characteristic of their new location. In contrast, anterior embryonic ectoderm showed some preference for definitive ectoderm differentiation following heterotopic transplantation. However, there was no evidence that the normal fate of tissue from the three regions studied could be explained by pre-existing mosaicism in the embryonic ectoderm.