Virgin female mice of two unrelated, highly inbred strains, BALB/cBy and C57BL/6By, seven of their derived recombinant inbred strains, and their F1 hybrid generations, were exposed either to an intermittently ringing bell or to a flashing, high-intensity light. The females were treated, before exposure, with 1·5 i.u. PMSG. After exposure, the number of ova per treated or per responding female and the percentage of responding females were counted. The matching strain distribution pattern and the test of a congenic line permitted characterization of a gene exerting a major effect on bell- or flash-induced ovulation. The gene is located in Chromosome 4 (LG VIII). We propose the symbol Bfo. There appeared to be one or more additional loci involved and possibly interaction between loci.