When two discrete stimuli are presented concurrently, with reinforcement contingent on a response to one of them (S+) but not to the other (S−), the redundancy of the situation may allow discrimination by the organism on the basis of information provided by either stimulus or both stimuli. In addition, when the two stimuli are in different positions, only one stimulus need be sampled on any trial to determine where to respond. The actual locus of control (S+, S−, or both) and the sampling strategy are not determinable within the standard acquisition paradigm. Answers to these problems were determined by introducing indifferent stimuli and nonstandard stimulus combinations to three dogs, each of which had previously acquired four simple olfactory discriminations. Results showed that each dog adopted an individual but highly consistent pattern of sampling and control.