We studied 24 healthy young male smokers to ascertain the medical safety of a highly effective smoking-abatement technique called rapid smoking. In comparison with results obtained after a 12-h cigarette fast or after normal smoking, statistically significant increases occurred after rapid smoking in heart and respiratory rates, systolic blood pressure, carboxyhemoglobin, and pH (p less than 0.05), while arterial PCO2, HCO3-, and serum [K+] fell (p less than 0.05). Rapid smoking produced alveolar hyperventilation in all subjects. In eight, arterial PO2 increased appropriately, but in 16, fell paradoxically (p less than 0.01); seven had arterial PO2 below 80 torr. Because this fall could represent ventilation/perfusion mismatch due to early bronchoconstrictive disease, in subjects with normal flow/volume loops and spirometry rapid smoking may be a simple, provocative screening test for early airway disease. Despite the changes produced there were no arrhythmias. Rapid smoking is safe for healthy subjects but should not now be used for higher-risk patients.