The effect of some sex hormones (estradiol-17 alpha), estradiol-17 beta, estrone, progesterone and testosterone) on the mechanical activity of rabbit left ventricle was studied in concentrations within the physiological range. Investigations were carried out during the first ten minutes after the introduction of the steroid. Only progesterone and estradiol-17 beta affect the isovolumetric pressure curve; the other steroids do not exhibit any effect. The effects of estradiol-17 beta and progesterone counterbalance each other. Estradiol-17 beta enhances the rate of pressure development and decreases the time to reach peak pressure. Progesterone decreases both the rate of pressure development and the time to reach peak pressure.