Phosphofructokinase (PFK) isozymes and their physiological significance have been the focus of extensive research. The majority of this work has been centered around the PFK isozymes of rat, human and rabbit tissues. Consequently, this review emphasizes these studies. Additionally, a review of PFK isozymes in chickens, mice, guinea pig, and pig is presented. The relationship of the properties of each PFK isozyme in different tissues to the rates of glycolysis and/or gluconeogenesis in those tissues is discussed where possible. Moreover, the contribution of the different PFK isoenzymes to alterations of the glycolytic rate in various tissues is discussed in relationship to variations in nutritional, hormonal, developmental or pathological status of the animal.