Nicotinic antagonists (piperidines and quinuclidines) reduce the susceptibility of early sea urchin embryos to agents evoking calcium shock.

1. Some nicotinic antagonists (piperidine and quinuclidine derivatives and bis-quaternary compounds) protect early embryos of the sea urchin Lytechinus pictus against a calcium shock evoked by ionomycin or a mixture of phorbol myristate acetate and nicotine. 2. Maximal protective potency was found for drugs that did not penetrate the plasma membrane. 3. Early sea urchin embryos have nicotinic acetylcholine receptors (nAChR) or nAChR-like structures localized on the cell surface that, apparently, take part in the control of Ca2+ influx.