The effect of the quinine derivatives, such as quinidin, cinchonin, cinchonidin, optochin, eucupin and vuzin, on the isolated uterus, Fallopian tube, round ligament and vaginal wall of the rabbit, is, in low concentration, to exhibit contraction, manifesting increased tonus and movement, through the stimulation of their musculatures, and in high concentration, to exhibit a depressing effect, owing to the paralysis of the muscles. Among these substances, quinidin, cinchonin and cinchonidin, which belong to the cuprein derivatives, and are closely related or isomeric in their chemical constitution to quinine, in a certain degree of concentration (0.005-0.01%) in which usually exert a marked stimulating effect, reverse the stimulating action of adrenalin by paralyzing selectively the motor fibers of the sympathetic nerves of those organs above mentioned. The stimulating action of adrenalin on the organs is reversed, with a pronounced depressive effect, by the subsequent addition of these drugs, and, in similar manner, the stimulating action caused by the quinine derivatives is also markedly depressed by adrenalin afterwards given. The concentrations of the drugs which are required for inducing the reversal of adrenalin action are similar, but in their consistency to cause such phenomena they show considerable variation. Cinchouin is the surest in this respect, followed by cinchonidin and quinidin. Quinine seems to be pretty uncertain. In striking contrast to these cuprein derivatives, the saturated hydrocuprein dervatives such as optochin, eucupin and vuzin have no power to cause the reversal of adrenalin action, except that optochin which has the least molecular weight among them, induces the reversal of adrenalin action very rarely.