Expansions and contractions of the auroral oval and substorm activity over a 10-day period were studied. Changes in latitude of the equatorward boundary of the diffuse aurora, scaled at 2000 CGT from DMSP photographs, reflect expansions and contractions of the oval. Hourly average and 5-min average IMF Bz component data and AE (11) data are examined relative to the expansions and contractions of the oval and to auroral substorm activity as seen in the photographs. The minimum-size oval occurs near 73¿ CGL at 2000 CGT. Over the range of Bz values examined, the oval expands or contracts by less than 2.5¿ within 102 min, which is the temporal resolution limit imposed by the DMSP satellite orbital period. As was previously reported, Bz, appears to control both the expansion and the contraction of the oval. Substorm activity accompanies both the expansion and the contraction. Substorms which occur during the oval expansion are more intense than the substorms which occur during the ensuing contraction. Also, the oval appears to be able to expand more rapidly than it can contract.