Responses of rhesus monkeys were reinforced by delivery of either a pentobarbital (4.0 mg/ml) solution or a vehicle (water) or saccharin solution under a concurrent signaled differential reinforcement of low rates 30-s schedule. After 30 s of no responding, the first response on the pentobarbital or saccharin spout resulted in the delivery of the appropriate solution and reset the timing on both spouts (i.e. a mutually exclusive choice). In the first experiment, the concentration of saccharin was gradually increased across sessions. As saccharin concentration increased, pentobarbital deliveries decreased and saccharin as well as total session deliveries increased. In a second experiment, pentobarbital and 0.24 (mg/ml) saccharin were made available under concurrent signaled differential reinforcement of low rates 30-s schedules which operated independently. Under these conditions responding on one spout had no consequences with respect to the other spout. The reduction of pentobarbital deliveries was substantially attenuated when the choice was not mutually exclusive.