Abstract 1. 1. The effects on sea urchin embryogenesis of continuous exposure of embryos, beginning at different times after fertilization, to chlorpromazine, a neurotransmitter and calmodulin antagonist, were examined. 2. 2. Embryos cultured in 100, 10, 5, 1 or 0.5 μM chlorpromazine beginning immediately after fertilization were arrested at pronuclear fusion, 4–8 cell, 32–64 cell, mesenchyme blastula, and early gastrula stages, respectively; 0.1 μM treated embryos developed normally. 3. 3. Although 1.0 μM chlorpromazine, added at any time prior to gastrulation, delayed cleavage and totally blocked gastrulation, embryos formed mesenchyme blastulae at the same time as controls but with fewer cells. 4. 4. Embryos cultured in 0.5 μM chlorpromazine beginning at any time prior to gastrulation developed normally to early gastrulae; however, further invagination of the future gut tube was blocked or delayed and spicule development was abnormal.