The modification of ciliary beating by neurotransmitters in sea urchin larvae at the four-armed pluteus stage was analyzed in terms of the direction of beating and fluctuation in the beat period. Application of dopamine to Pseudocentrotus depressus causes the cilia to turn their beat plane but retain its characteristic planar feature up to the complete 'reversal' of the beat direction. This new type of response was termed the 'beat-plane turning response'. It was also found that neurotransmitters, especially dopamine and serotonin, can modify the length of the beating cycle in P. depressus and Hemicentrotus pulcherrimus. Dopamine decreased and serotonin increased the beat frequency averaged over the ciliated epithelium with the standard deviation from the mean increasing in the presence of dopamine and decreasing with serotonin. The beat-period fluctuation and its modification suggested by this observation was confirmed from measurements of the beating of individual cilia in the presence or absence of these neurotransmitters. Further analysis of the correlation between angular velocity and beat period indicates that variation in the beat period is not controlled by the same processes as those that modulate angular velocity. These findings in sea urchin larvae suggest that both the stability and the direction of ciliary beating is under nervous control.