We analyzed measurements of the total carbon monoxide (CO) content in the atmosphere in the region of St. Petersburg (59.88°N, 29.83° E; 20 m above sea level) in the period from 1995 to 2009. The average annual behavior for the entire measurement period has a maximum in February–March and a minimum in July with an amplitude of ~20%. In the absence of strong forest fires in the European part of the Russian Federation and Siberia, the annual minimum of the total CO content is usually recorded in August–September. In winter 1995–2009 (November–January), there was a decrease in the total CO content with a gradual shift in the maximum of the annual behavior from January (1995–1999) to February (2000–2004) and March(2005–2009). The total CO content in January–February 2009 was ~20% lower than the multiyear average level. Estimates of the linear trend for the maximum, minimum, and average values for the period of 1996– 2009 showed an absence of statistically significant longterm changes in the total CO content. A sp