A study of the correlation of the geosynchronous magnetic field with interplanetary magnetic field (IMF) Bz and the solar wind dynamic pressure (Pd) is presented. Hourly averages of 5 years of GOES 6 and 6 years of GOES 7 observations are correlated with IMF Bz and Pd. As previously reported, increases in Pd enhance geosynchronous Bz on the dayside, most strongly around noon, but depress it on the nightside, most strongly around midnight. This has been interpreted in terms of increases in the cross-tail and magnetopause currents. Our study shows that the dayside geosynchronous magnetic field decreases with IMF Bz, particularly during periods of southward IMF. During periods of northward IMF, this trend continues but at much slower rate. The results of a multiple regression analysis of GOES Bz as a function of IMF Bz and Pd during periods of northward IMF show that roughly 30% of the IMF Bz uniformly penetrates the geosynchronous Bz. In contrast, during periods of southward IMF, the effect of IMF Bz on geosynchronous Bz is nonuniform and much larger at all local times, especially near dawn and dusk, apparently because of enhanced cross-tail current. During periods of southward IMF, geosynchronous Bx depends most strongly on IMF Bz near dawn, dusk, and midnight. The dawn and dusk correlations can be attributed to the enhanced region 1 Birkeland currents, and the midnight correlation can be attributed to enhanced cross-tail current. Geosynchronous By has the best correlations with IMF Bz near 2000 LT and 0400 LT, which can be attributed to enhanced region 1 Birkeland and/or cross-tail currents. Also, we show that these magnetic field perturbations can be interpreted in terms of fast rarefaction waves and merging at equatorial regions.¿ 1997 American Geophysical Union