1. External cycles may influence biological functions by entraining their endogenous circadian oscillation or by directly influencing the function measured. In order to distinguish between these two possible effects it is necessary to record the free-running period of the circadian oscillation under constant conditions, before and after the exposure to an external cycle (Fig. 4) since otherwise the results might be obscured by masking effects (Figs. 1-3). Here only synchronization of the circadian cycle is called a Zeitgeber effect. 2. Whether the circadian oscillation is entrained by a Zeitgeber depends on the difference between the spontaneous circadian period and the period of the Zeitgeber, and the strength of the Zeitgeber (Fig. 5). The strength of the Zeitgeber also influences the duration of resynchronization after a phase shift of the Zeitgeber oscillation, and the phase relation between circadian cycle and Zeitgeber cycle during entrainment. These conclusions, which are derived from the assumption that the circadian periodicity is a self-sustained oscillation in the technical sense, have been experimentally verified for biological circadian oscillations (Figs. 6 and 7). 3. The statement that certain types of factors act as Zeitgebers and others do not, is not meaningful in such general terms, as shown by the experimental evidence. There are only different strengths of Zeitgeber modalities. However, in this respect one would expect large interspecific and even intraspecific differences, depending on organization and biology of the different organisms. An example for interspecific differences in Zeitgeber strength of light cycles is given (Fig. 9). 4. Light, temperature (Table 1 and 2), acoustical signals (Table 3) and electrical fields have so far been demonstrated to act as Zeitgebers. It is suposed that many other Zeitgeber modalities will be found, which will be different for different groups of organisms. The only valid generalization so far seems to be that temperature cycles are strong Zeitgebers in poikilothermic organisms, but only very weak ones in homoiotherms (Table 1 and 2). 5. It is mentioned that Zeitgeber effects may also contribute to the internal synchronization of different circadian oscillations within the organism.