Nucleonic intensity data from the north and south polar stations Thule and McMurdo are analyzed to determine the nature of periodicities in the north-south asymmetry of relativistic galactic cosmic rays. The 24-hour average intensity differences of individual days range from 0 to ¿1.5%, and the most prominent time variation is recurrence tendency with a period consistent with the 27-day solar rotation period. A recently developed procedure for evaluating the statistical significance of quasipersistent data has been utilized to establish the reality of the 27-day signal disclosed by superposed epoch analysis of the north-south asymmetry data. Analyses of various components of the interplanetary magnetic field reveal that the IMF component B¿&mgr; (parallel to the nominal garden hose direction) also displays a 27-day variation with maximum amplitude. Furthermore, a significant relationship between B¿&mgr; and the north-south asymmetry is established by Chree analysis. On the basis of this relationship, it is suggested that the 27-day variation in the north-south asymmetry arises from the B¿&mgr;¿∇¿(n) drift, where ∇¿(n) represents the radial gradient of cosmic rays.