A microbath.

A microbath, with a volume of less than 50 cubic millimetres, for testing the effects of drugs on small pieces of smooth muscle, is described. Drugs are applied automatically in any desired order and the record of their effects is amplified electrically. In this apparatus the response of small pieces of rat uterus to drugs is apt to be nearly all-or-none. Contractions of the muscle from a single ring of guinea-pig trachea were recorded. A fall of pH was found to reduce the effect of 5-hydroxytryptamine on this muscle while leaving the effects of tryptamine and acetylcholine unchanged.