Ambiental levels of formaldehyde and acetaldehyde were measured at three sites in the city of Londrina: at the central bus station, downtown (night and day profiles) and in a rural area, during the summer and winter seasons of 2002. Atmospheric aldehydes were collected using C18 Sep-Pak cartridges coated with 2,4-DNPH and analyzed by HPLC-DAD. At the central bus station, the measured mixing atios for formaldehyde and acetaldehyde ranged, respectively, from 6.17 to 10.43 ppbv and from 0.49 to 2.12 ppbv. Downtown, during the summer, formaldehyde levels ranged from 2.07 to 5.65 ppbv, and acetaldehyde levels ranged from 1.05 to 5.06 ppbv. During the winter, the daytime concentrations for formaldehyde and acetaldehyde were higher than nighttime levels. Formaldehyde and acetaldehyde mixing ratios ranged from 3.24 to 9.46 ppbv and from 2.81 to 10.83 ppbv, respectively. At the rural site, formaldehyde and acetaldehyde mixing ratios ranged from 0.64 to 1.41 ppbv and from 0.10 to 0.87 ppbv, respectively. The acetaldehyde/formaldehyde ratios were compared with values from other Brazilian cities.