The recombination rate in meiosis between the mouse X and Y chromosomes was analyzed. Mice heterozygous at two pseudoautosomal alleles, the steroid sulfatase gene and the Mov-15 provirus marker, were crossed. The provirus in the Mov-15 transgenic mouse strain had been previously shown to be carried in the pseudoautosomal region of the sex chromosomes. Recombination frequencies were shown to be 7-fold higher in this region in male meiosis than in female meiosis. Three-point crosses indicated the occurrence in male meiosis of double recombination events in the pseudoautosomal region, with little or no crossover interference, which is in marked contrast to observations made on the similar region of the human sex chromosomes. This result is contrary to a previous model, which predicted a single crossover event in male meiotic pairing of mammalian sex chromosomes.