Serotonin levels in plasma have been found to differ significantly between inbred strains of mice. For example, the level in the C57BL/6By strain is more than twice that in BALB/cBy (Table 1). It was to determine the genetic basis of this difference that we have turned to a new genetic device, the recombinant inbred (RI) strains. RI strains are derived from the cross of two unrelated, but highly inbred progenitor strains, and then maintained independently, from the F2 generation onwards, under a regimen of strict inbreeding. The procedure genetically fixes the chance recombination of genes in generations following the F1 generation, although in ever decreasing amounts as full homozygosity is approached. The resulting battery of strains can be looked upon, in one sense, as a replicable recombinant population. The utility of such strains for analysing gene systems is described elsewhere (Bailey, 1971). Here we report the application of