Homotopic and isochronic transplantation of the alar plate of the rhombomeres 7 and 8 was performed between chick and quail embryos at the stage of 10-14 somites. Analysis of the graft derivatives in 12-day-old chimeric embryos by means of the quail nucleolar marker showed that the ipsilateral inferior olive is formed from the transplanted neuroepithelium. In all embryos some cells originating from the graft were also found scattered throughout the contralateral inferior olive. The present results demonstrate that the inferior olive derives from the alar plate of the rhombomeres 7 and 8 and support the notion that a small contingent of inferior olivary neurones crosses the interolivary commissure during development.