Rhythmic springs (ebb and flow springs, intermittent springs, potajnice) belong to the group of springs which appear exclusively in karstified terrains. The paper describes various types of rhythmic springs and gives their classification. It also develops a mathematical model for the functioning of this type of springs based on the principle of recharge and emptying of the underground reservoir through siphon action. Applying this model, according to the observed hydrographs of some rythmic springs in Yugoslavia, the paper explains in detail the structure of the underground reservoir located in the karst.