Choline esters of polyenoic fatty acids block cleavage divisions of sea urchins and evoke the formation of one-cell multinuclear embryos. If the fatty acids AA-Ch or DHA-Ch are added at the mid or late blastula stage, many cells are extruded, forming extra-embryonic cell clusters near the animal pole of embryos or larvae. Both effects are prevented by dimethylaminoethyl esters of polyenoic fatty acids (AA-DMAE or DHA-DMAE) or their 5-hydroxytryptamides. Nicotinic acetylcholine receptor antagonists, imechine, d-tubocurarine or QX-222 provide partial protection against AA-Ch or DHA-Ch. The organophosphate pesticide, chlorpyrifos, or a combination of (-)-nicotine + phorbol 12-myristate 13-acetate, also evoke the mass extrusion of transformed embryonic cells at the animal pole of larvae. These effects are similarly antagonized by AA-DMAE, DHA-DMAE, or fatty acids 5-hydroxytryptamides. Taking together, these results suggest that AA-Ch and DHA-Ch act on sea urchin embryos and larvae as agonists of acetylcholine receptors, whereas AA-DMAE and DHA-DMAE act as antagonists. The ability of fatty acids 5-hydroxytryptamides to prevent the effects of AA-Ch or DHA-Ch may be due to restoration of the normal dynamic balance of cholinergic and serotonergic signaling during cleavage divisions and gastrulation.