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Two parameters for scheduling aversive stimulus presentations were studied systematically by specifying concurrent and independent probabilities of electric shock delivery for the occurrence and for the non-occurrence of a lever-press response. After preliminary training on a free-operant shock-avoidance schedule, 16 rhesus monkeys were divided into four groups, each group being assigned one shock distribution on a continuum from fixed interval to a widely ranging variable interval. Within groups, each subject was successively exposed to three values of response-dependence of shock delivery on a continuum from response-independent shock to complete dependence of shock on response occurrence ("punishment"). Introduction of shock following avoidance training produced initial response facilitation followed by suppression. Responding during both the facilitation and suppression periods was maximal when the shock schedule was periodic and response independent. Responding decreased as the inter-shock intervals were made more variable across groups, and as shock delivery was made increasingly response dependent within individual subjects.