MACHUPO virus, the aetiological agent of Bolivian haemorrhagic fever, is an ether-sensitive RNA virus immunologically classified as a member of the Tacaribe group of “arboviruses”1–3. Repeated isolation of this virus from the cricetine rodent Calomys callosus4, the demonstration of chronic virus infection with persistent viruria in this animal5, and the inability to recover virus from a large number of arthropods associated with C. callosus6, led to serious doubt that biological transmission by arthropods was an important mechanism in the natural maintenance of Machupo virus. To facilitate inquiry into this problem, a laboratory colony of C. callosus was established7. We report here preliminary work on the dynamics of experimental infection which demonstrates that Machupo virus regularly induces immune tolerance in Calomys infants and in a portion of animals infected as adults.