Carbon monoxide (CO) and ozone (OA3) concentrations have been observed in the Brazilian Amazon region, at a site strongly affected by biomass burning (Cuiab¿, 16¿S, 58¿W). Time variations are described for the first long-term program of studying the effect of biomass burning on O3 and CO over a complete seasonal cycle, including the seasonal maxima of 1987 and 1988. Concentrations of O3 are measured continuously, and CO samples are collected three times a week. In order to obtain elements for comparison, an identical observational program was maintained at a site totally outside of the direct influence of biomass burning (Natal, 6¿S, 35¿W). The biomass burning contribution to the Cuiab¿ concentrations of CO and O3 is very large. Diurnal maximum concentrations exceeded 90 ppbv (parts per billion by volume) O3 in 1987 and 120 ppbv O3 in 1988, in September. For the wet season, the monthly average ozone concentration in March--April is about 10 ppbv. During the month of maxima, September, the O3 concentration average was 41 ppbv for 1987 and 71 ppbv for 1988. The CO concentrations are about 90 ppbv in the wet season. In September, 460 ppbv and 660 ppbv of CO were observed for 1987 and 1988, respectively. At Natal the seasonal variation is of the order of a factor of 2. For the biomass burning site this factor is 4 for 1987 and almost 7 for 1988. In contrast, during the wet season, the concentrations of CO and O3 at both stations are about the same.