1. The cardiovascular responses to passive upright tilting were determined in healthy human subjects of various ages using entirely 'non-invasive' techniques. Cardiac output was determined by a single breath method and arterial blood pressure by an automatic sphygmomanometer. 2. Steady-state responses were achieved within 4-6 min from the onset of the tilt and were maintained for at least 15 min. 3. The reproducibilities of the responses to tilting by 60 degrees, expressed as two standard deviations of the differences between responses on two occasions (mean responses in parentheses), were: cardiac output +/- 0.70 (-1.41) litre/min, heart rate +/- 7.6 (+14.7) beats/min, systolic blood pressure +/- 15.5 (+1.0), and diastolic blood pressure +/- 13.1 (+13.6) mmHg. 4. Tilting by 20 degrees resulted in no significant responses but between 20 degrees and 60 degrees responses were linearly related to the angle of tilt. 5. Supine values and responses to tilting by 60 degrees were compared in subjects in four age groups between 20 and 80 years. With increasing age, the supine values of cardiac output declined and those of arterial blood pressures increased. Responses of heart rate, cardiac output and diastolic pressure declined by increasing age. These results, obtained from a healthy population, provide reference values for comparison with individuals who may have deficient postural responses.