In the biggest forest of West Berlin, the Grunewald, with an area of 3100 ha, the Yellow-necked field mouse (Apodemus flavicollis) is the most abundant vertebrate. From 1952 to 1973 I collected owl pellets (9/10 of which were Tawny owl pellets) from the West Berlin area, and found in them 35041 vertebrates. During the investigation it was found that the actual percentage of Apodemus flavicollis in the total prey from the nesting Tawny olws (Strix aluco) in the Grunewald showed a noticeable peak every 3 years, fluctuating between 36.2 to 40% of the total take. In both the following years the actual percentage falls on a regular basis to approximately half (17.6 to 23.7%). Investigation of the breeding longeared owls (Asio otus) in the Grunewald gave the same picture. Only in 1973, the last year of the investigation, did a break in this regularity occur.One of the diagrams representing these 21 years of investigation (see Fig. 3) shows this out-of-the-ordinary regularity over the whole period. It is particularly noticeable that all peaks of the curve lie at nearly the same height (36-40%), and also that the actual percentage values of the low point years show only slight variation, (17.6-23.7%). One parallel study over 11 years in another West-Berlin forest area showed similar results.From these results it can be concluded that the regulation of population size of the Yellownecked field mouse pass through a rhythmical 3-year cycle which is affected neither by the weather (climate) nor by food availability, but is definitely regulated by physiological intraspecific events.