A systematic study was made of several nonhuman primates to learn more about their relative usefulness for studies of sleep. Species studied included the Guinea baboon (Papio papio), Kenya or yellow baboon (P cynocephalus), olive baboon (P anubis), sacred baboon (P hamadryas), vervet monkey (Cercopithecus aethiops), rhesus monkey (Macaca mulatta), bonnet monkey (M radiata), crab-eating monkey (M fascicularis), patas monkey (Erythrocebus patas), chimpanzee (Pan troglodytes), mongoose lemur (Lemur mongoz), black lemur (Lemur macaco fulvus), and bushbaby (Galago senegalensis). Comparisons were made of electroencephalographic activity, states of vigilance, and responses to sleep-waking drugs. The results showed that there were major similarities among the genera studied as well as individual intrageneric and intraspecies differences. It was concluded that the chimpanzee, olive baboon, and rhesus monkey were the best species for comparative studies, and that the rhesus monkey was the best single model because of its well defined sleep organization and ease of handling and housing.