The apparatus described consists of a 50 cc. muscle chamber, a 100 cc. reservoir for refilling the muscle chamber, and a pump for oxygenating and circulating the immersion medium. This apparatus permits circulation of the immersion medium (Locke''s solution) without disturbing the immersed tissue, and change of the solution in the muscle chamber without exposing the tissues to the air. Rhythmical changes in the muscular activity of the uterine horn in the rat were found closely to parallel orderly changes in the vaginal smears made in different phases of the oestrous cycle. The peak of muscular activity occurs in stage III (late oestrus) ; the least activity immediately precedes the most active stage. Minute doses of pituitrin transformed slight muscular activity into maximal contractions.