Two studies were conducted to determine the effect of blocking filter vents on carbon monoxide (CO) exposure under ad lib smoking conditions. In Study 1, 12 daily cigarette smokers smoked cigarettes from the brands Now (1 mg tar by the FTC Method) and Marlboro Lights (10 mg tar) under each of two vent-blocking conditions (unblocked and finger blocked). Blocking filter vents with fingers led to an 85% increase in CO for the brand Now, but had no added effect on CO exposure from the Marlboro Lights. In Study 2, another 12 daily cigarette smokers smoked cigarettes from each of four additional brands: Carlton (1 mg tar), Now (2 mg tar), Virginia Slims Ultra-lights (5 mg tar), and Virginia Slims Lights (8 mg tar). Blocking filter vents with the lips caused all four brands to produce equal CO exposures. Blocking vents increased smokers' exposure to CO by 239% when smoking Carltons and by 44% when smoking Nows. No significant increases in CO with blocking were found for either of the Virginia Slims brands. These results suggest that the degree to which a brand is ventilated determines whether that brand is susceptible to increased CO yields as a result of vent blocking.