Combined doses of independent full agonists were tested on the rat isolated vas deferens. When the organ was contracted by a maximum dose of adrenaline, acetylcholine, histamine or serotonin, an additional con traction could be elicited through barium chloride. This was taken as an indication of an effector reserve which can be exhibited in the organ after a full response through a given receptor system. It was shown that this effector reserve can be only partially evaluated through the use of agonists others than barium chloride. The results were confronted with those expected from earlier studies on functional synergism, and discussed in relation to the relative responsiveness of pharmacological receptor systems, and of the occupation theory of mode of drug action. It was concluded that the data cannot be totally explained through this theory. A modified model based in multistep sequential reactions was suggested as an attempt to explain the results obtained.