ABSTRACT An account is given of the responses of Metridium senile to mechanical stimulation. The responses resemble those described by Parker in M. marginatum. The chief protective response is due to contraction of the longitudinal mesenteric muscles and not to contraction of the sphincter as in Calliactis. A simple apparatus for stimulation of Actinozoa is described. The behaviour of Metridium to electrical stimulation resembles that of Calliactis. The response does not depend upon the intensity of individual shocks but on their number and frequency. As with Calliactis, Metridium obeys the rule that a single stimulus produces no effect. The longitudinal mesenteries of Metridium resemble the sphincter of Calliactis remarkably closely in their properties. As in Calliactis, graded responses can be produced in Metridium by varying the frequency of stimulation. The longitudinal mesenteries require the highest frequency for a facilitated response. The parietals require a lower frequency. The sphincter requires a relatively high frequency of stimulation but its action is delayed owing to its slow rate of contraction. The graded responses of different muscles to stimulation are controlled by facilitation. They are not connected with differences in threshold. Temperature has a profound effect on the responses of actinozoans. Both contraction and relaxation are slowed at low temperatures (Q10 = 2). The facilitating effect of a stimulus endures more than three times longer for a fall in temperature of 10° C. Temperature thus greatly influences the rate of decay of the facilitation process.