As part of the North Atlantic Regional Experiment (NARE), measurements of O3 and CO at five surface sites were made from July 1991 to January 1995. The investigation of the variabilities and correlation of O3 and CO presented here indicates that the seasonal cycles of the medians and the means of O3 and CO are qualitatively similar to the cycles observed at other northern midlatitude sites. The signature of O3 produced from anthropogenic precursors is clearest in the spring at the Azores and in the summer at Sable Island. The influence of the natural stratospheric O3 source is apparent at Sable Island, particularly in the spring. At all sites the variability of CO throughout the year is dominated by episodes of pollution transport. The slopes of the monthly O3-CO correlations in the summer in Atlantic Canada and the spring in the Azores are quite uniform at 0.3 to 0.4. However, individual pollution transport events often have larger (≤1.0) slopes, which indicate significantly different net O3 production efficiencies between episodes. The average slope of O3 versus CO at Sable Island in the winter for moderate pollution transport events (CO≤180 ppbv) is -0.28, which indicates that the titration of ambient O3 by emitted NO with little if any photochemical O3 production dominates the O3 chemistry over eastern North America in winter. Diurnal cycles driven by photochemistry are observed in the summer for both O3 and CO at the Azores (net loss) and Sable Island (net production.) These observations are consistent with the work of Duderstadt et al. who find positive net photochemical O3 production at Sable Island, and with the modeling of Atherton et al. <1996> who find a region dominated by photochemical loss of O3 and CO in the central Atlantic. ¿ 1998 American Geophysical Union