Isolated males of the ensiferous Orthoptera stridulate under natural conditions with regular intervals. The main subject of the work described in this paper is an attempt to clarify the interrelationship between internal and external conditions activating this habit. To this end observations were made of several species both in the field and in the laboratory under controlled conditions, in order to establish the relation between the occurrence of stridulation - commencement, duration and intensity - and the prevailing environmental factors. During the work was established the following working hypothesis which covers all the observations: The stridulation is an effect of a continuously produced sensitizing agent which at a certain level releases the activity; the rate of increase of sensitization is correlated with temperature. The threshold of release is higher at high illumination and at high temperature than in darkness and at low temperature. During the activity' the sensitization is reduced to a low level. Reduction in illumination releases also spontaneous motoric activity and, at least in G. domesticus, also feeding. These activities prevail over stridulation, so that the song will first commence after a certain lapse of time, the period of latency. At the beginning of stridulation there is an increase in intensity during an interval of similar duration, the period of increment.