Abstract.Eight male habitual smokers smoked two cigarettes over a 20‐min period following a 12‐h period of abstinence. Antecubital venipuncture was performed immediately before, immediately after, and 55 min and 2 h after smoking had ceased. At these times, the mean values (± SD) of collagen‐induced platelet aggregation were 45 ± 5, 68 ± 5, 59 ± 6 and 52 ± 5 chart units, respectively, while the corresponding values for the mean platelet aggregate ratio were 0.91 ± 0.01, 0.82 ± 0.03, 0.87 ± 0.02 and 0.90 ± 0.02, respectively. Mean collagen‐induced platelet aggregation was significantly (P<0.005) higher immediately after, and 55 min and 2 h after smoking. The mean platelet aggregate ratio was significantly (P<0.001) lower immediately after and 55 min after smoking. Correlation coefficients between the concentration of nicotine in each of the 24 plasma samples obtained after smoking and the corresponding values of collagen‐induced platelet aggregation and the platelet aggregate ratio were 0.41 (P<0.05) and −0.50 (P<0.02), respectively. It is concluded that when habitual smokers abstain from smoking overnight, a 20‐min period of cigarette smoking may enhance platelet aggregabil