SUMMARY: Plasma corticosterone levels were determined fluorometrically in mice of two unrelated highly inbred strains, C57BL/6By and BALB/cBy, and in seven of their derived recombinant-inbred strains as well as their F1 hybrid and backcross generations necessary to arrive at a genetic model for plasma corticosterone levels. It was concluded that the simplest genetic model, and one which fits the experimental results, was one which assumed that plasma corticosterone levels are controlled genetically by two loci with the epistatic interaction indicating dependency of pathways of action for the two genes.