To determine the effects of age on the pulmonary circulation at rest and on exercise we analysed the results of right heart catheterization studies performed in 125 asymptomatic subjects aged 14-68 years, who were healthy or had indispositions which did not impair cardiac or pulmonary function. Age accounted for less than 10% of total variation in resting values of right atrial, pulmonary artery and wedge pressures, and of cardiac output. The pulmonary artery-wedge pressure gradient and flow resistance at rest significantly increased with age. On exercise there were significant increases with age in right atrial, pulmonary artery and wedge pressures, pulmonary to wedge pressure gradient and flow resistance, but cardiac output was not influenced by age. Pulmonary circulation variables at rest are mainly influenced by sex and size, but during exercise significant effects of age are apparent.