In order to study the relationship between pharmacologically induced hippocampal theta rhythm and resistance to extinction of instrumental responding, three groups of rats were trained on a discrete-trial FR 10 schedule of reinforcement, one group receiving injections of physostigmine (producing hippocampal theta rhythm) and the other two groups saline injections. This was followed by a noinjection phase for all three groups during which the schedule of reinforcement remained unchanged. In the extinction phase the drug-acquisition group and one of the saline acquisition groups received saline injections, while the remaining group received drug injections. Following extinction allSs went through reacquisition and a second extinction without any injections. The findings were that if physostigmine was administered during acquisition resistance to subsequent extinction was enhanced, but if it was administered during the extinction phase extinction was accelerated. The second acquisition and extinction did not yield any significant drug effects.