Abstract 1. 1. Lateral ciliary activity was studied on intact and isolated gill cell preparations in the mussel, Mytilus edulis. 2. 2. It has been previously demonstrated by the senior author that the lateral cilia are under the control of both excitatory and inhibitory axons present in the branchial nerve. Furthermore, it was shown that low frequency [5 Hz; neurotransmitter released is serotonin (5-HT)] and high frequency (50 Hz; neurotransmitter released is dopamine (DA)) nerve stimulation activated cilio-excitatory and/or cilio-inhibitory axons. 3. 3. This study confirms the above findings and in addition shows that perfusing intact and isolated gill cell preparations with salyrgan (Ca 2+-ATPase poison) in the presence of external Ca 2+ is also cilio-inhibitory. Salyrgan potentiates the cilio-inhibition produced by high frequency nerve stimulation/DA. It antagonizes the cilio-excitation that occurred after low frequency nerve stimulation/5-HT. The addition of lanthanum to the salyrgan perfusate enhances the cilio-inhibitory response of the latter probably by more effectively stopping the Ca 2+-pump. 4. 4. It is postulated that salyrgan effects the Ca 2+-pump of lateral cells by slowing or stopping (with addition of lanthanum) the pump so that external Ca 2+ leaks across the cell membrane. A subsequent rise in internal Ca 2+ concentration produces lateral ciliary arrest.