Recent studies suggest that the tissue/organ proportions of 6-phosphofructo-1-kinase (PFK) subunits from diverse strains of rat may be drastically different. To test this possibility rigorously, the PFK isoenzyme populations and subunit contents in muscle, liver, brain and heart were examined in the following strains: Wistar, ACI, Long Evans, Norway Brown and Wag/Rij. Regardless of the strain, adult muscle possessed only the M-type subunit; adult liver contained predominantly the L-type subunit as well as M-type and C-type subunits; and the adult brain and heart exhibited all three subunit types.