Cerebrorubral (CR) and cerebellorubral (CBR) afferents are localized to distal dendrites and somata of red nucleus (RN) neurons, respectively, in adult cat. To test if this synaptic site segregation is established by a sequential arrival of the two afferents, the development of CR and CBR projections was studied. CR axons arrived in the RN at embryonic day (E) 50 or 51, while CBR axons entered the RN before E35. These results suggest the possibility that arrival of CBR axons before CR axons leads to the segregation of synaptic sites.