By elution with a discontinuous gradient from QAE-Sephadex, the rat muscle 6-phosphofructo-1-kinase (PFK) isozyme (M4) and the major rat liver PFK isozyme (L4) could be completely separated. Subjecting heart supernatant fluid to this treatment indicated that all of the heart PFK activity was found in the first wash, where M4 eluted, indicating little, if any, L4 was present. However, about 50% of the heart PFK activity was immunoprecipitated by L4 anti-IgG demonstrating the presence of the L-type subunit. A purification procedure was developed which yielded an enzyme preparation of high specific activity and resulted in a recovery of 50% to 60% of the original PFK activity. Three proteins were detected in this PFK preparation that exhibited apparent mol wt of 87 500, 85 000 and 80 000. The 85 000 and 80 000 Dalton components corresponded to the subunits of M4 and L4, respectively. The third protein was thought to be a distinct subunit (C-type) since it exhibited the highest molecular weight and was present in an exhaustively washed immunoprecipitate of purified heart PFK. From seven different heart PFK preparations, the relative distributions of the L-, M-, and C-type subunits were 0.80 +/- 0.1, 4.5 +/- 0.7, and 0.7 +/- 0.1, respectively. A comparison of the kinetic properties of L4, M4, and purified heart PFK isozymes clearly demonstrated that all three preparations exhibited different regulatory properties. In ventricular and atrial preparations the total PFK activity and the relative amounts of each subunit were drastically different suggesting regional differences between the distributions of PFK isozymes and consequently in regulation of PFK activities and glycolysis.