Effect of calcium ionophore A23187 on the sensitivity of early sea urchin embryos to cytotoxic neuropharmacological drugs." The ability of cytotoxic neurochemicals (indole and amphetamine derivatives) to block first cleavage division in the embryos of the sea urchin Arbacia lixula abruptly increases when the embryos are incubated in calcium-free seawater and decreases when the external Ca concentration is raised up to 46.4 mM. Sensitivity of the embryos to these drugs decreases also in the presence of the Ca-ionophore A23187. It is suggested that Ca ions are involved in the realization of physiological effects of ""prenervous"" neurotransmitters whose presence in early sea urchin embryos was shown by us earlier."