Quantitative trait loci (QTL) approach was used in CXB recombinant inbred mice for preliminary identification of candidate regions on the mouse genome that influence sleep. The only provisional QTLs identified were associated with paradoxical sleep (PS). PS during the light period was associated with markers on chromosome 7 between 7 and 20 centimorgan from the centromere. For PS during the dark period, a single QTL was identified on chromosome 5, near the Clock gene. The 24 h amount of PS was influenced by markers on chromosomes 2, 17, and 19. This first QTL mapping study strongly suggests that a complex behaviour like PS can be controlled by only a few genes.