SummaryA group of 22 volunteers who smoked daily more than 20‘high’nicotine (0.8 to 1.1 mg) Canadian cigarettes were switched to lower yield brands in two stages aver an eight‐week period. The control group (six subjects) switched brands with nicotine yields within ± 0.1 mg of their usual brand while the treatment group smoked reduced yield brands (first stage, 33% reduction; second stage 61% reduction).all averages for levels of blood carboxyhemoglobin, plasma thiocyanate and serum cotinine were found to change significantly from week to week but there was no discernible difference between the treatment and the control group in week to week pattern. Although the majority of smokers in this study did not increase their exposure to cigarette smoke by smoking low yield cigarettes, it can not be said that the switch resulted in ‘safer