Progesterone causes a rapid increase in the intracellular free calcium level in fully grown amphibian oocytes. When albino Xenopus laevis oocytes were microinjected with the Ca-specific photoprotein aequorin, the calcium-induced luminescence from this protein increased at 40-60 sec after the addition of progesterone and returned to the control level within 5-6 min. No further change in the aequ...