We have developed a novel, eight-chamber superfusion system that is suitable for a variety of applications involving the study of both contraction and relaxation of smooth muscle preparations, and the effect of agents that interfere with these actions. The system allows electrical stimulation of preparations, and thus neuronally mediated responses and agents that interfere with neurotransmission may also be studied. To demonstrate some of the applications of the system, we have evaluated both spasmogenic and spasmolytic agents on the guinea-pig isolated tracheal strip preparation. The potency and the times for onset and offset of action of the spasmogens, acetylcholine, histamine, and prostaglandin F2 alpha, and the spasmolytics, isoprenaline, clenbuterol, salbutamol, papaverine, N-ethylcarboxamide adenosine, theophylline, and verapamil, have been investigated. The spasmolytic agents have been tested against both prostaglandin F2 alpha-induced tone and electrically induced contractile responses of the guinea-pig trachea. This superfusion system has several advantages over previously described superfusion or immersion techniques. It is compact and allows simultaneous study of up to eight preparations. It is suitable for a wide range of tissues, and the use of this system avoids the necessity of repeatedly washing drugs from organ baths. However, one of the most important applications of the system is its use in the study of rates of onset and offset of drug action. We believe, therefore, that this system represents an important alternative to the classical organ bath for in vitro pharmacological experimentation.