A behavioral tolerance-time procedure for measuring hypoxia-induced disruption of avoidance behavior was developed using squirrel monkeys (Saimiri sciureus) under a Sidman avoidance contingency. Hypoxic atmospheres were acutely and repeatedly presented to highly trained monkeys during avoidance sessions with 5 min of normoxia between presentations of hypoxia. Behavioral tolerance was defined as the period from the initiation of hypoxia to the occurrence of the first shock during each hypoxic presentation. Parametric studies demonstrated that reliable behavioral tolerance times could be obtained using a 5-sec response-shock interval and a 7% O2 atmosphere. Acetazolamide, a drug previously shown to be beneficial for several types of functioning under hypoxic conditions, markedly lengthened behavioral tolerance time, suggesting a valid model for evaluating drug therapies. Tolerance time is often used as a dependent variable in the study of the effects of hypoxic environments on living organisms. The definition of tolerance covers the interval from initiation of the hypoxic condition to an endpoint that is identical for all Ss. Physiological and pharmacological investigations have often used death as the endpoint, although other easily measured endpoints are loss of consciousness and drops in body temperature. Death has the obvious disadvantage of preventing repeated measurements using the same S. Loss of consciousness and large drops in body temperature may have gross nonreversible or long-term effects (Fregley, 1954). A preferable method of measuring tolerance to hypoxia would be one which left the animal relatively intact, so that reliable repeated measurements could be taken. An alternative measure involves a behavioral definition. Such a definition should involve an endpoint which is meaningful in terms of behavior and in terms of the environmental variable being tolerated. A behavioral schedule which fulfills these requirements is Sidman (nondiscriminated) avoidance (Sidman, 1953). This schedule involves the organism's repeatedly postponing the occurrence of an aversive stimulus,