We studied 67 healthy males aged 20-30 years, 29 of them non-smokers and 38 smokers. In smokers, forced expiratory volume (FEV), 1 s forced expiratory volume (FEV1) and peak flow (PF) were significantly lower than in non-smokers, while the FEV1/FEV ratio, flows and mean transit time (MTT) compared well. MTT appeared directly related to age and negatively to FEV1/FEV, with no difference between the two groups, but MTT was directly related to FEV in non-smokers and negatively in smokers. Multiple regression analysis, MTT = a + b(FEV)+c(age), showed that parameters b and c were significantly different between smokers and non-smokers and, in the former, parameter b was negative. In non-smokers the effects of age and FEV on MTT were roughly the same, but in smokers MTT was influenced above all by age, probably because MTT measured the effects of continued smoking.