1. Carotid baroreceptors were stimulated by application of a subatmospheric pressure to a chamber fitted round the anterior and lateral aspects of the subject's neck (neck suction). Pulse interval and heart rate were determined from an electrocardiogram, cardiac output by a single-breath method and arterial blood pressure by an automatic sphygmomanometer. 2. The maximal prolongation of the pulse interval, determined during held expiration, occurred within 2-3 s from the onset of the neck suction. All the measured variables were in steady states between 2 and 3 min from the start of neck suction. 3. Neck suction at -10 mmHg resulted only in an immediate change in pulse interval. All variables changed approximately linearly with the magnitude of the neck suction between -10 and -40 mmHg. 4. The reproducibilities of the responses to neck suction at -30 mmHg, expressed as two standard deviations of the differences between responses on two occasions, were (mean responses in parentheses): immediate pulse interval, +/- 32 (+236) ms; steady-state heart rate, +/- 2.5 (-6.5) beats/min; cardiac output +/- 0.14 (-0.59) l/min; systolic and diastolic blood pressures, +/- 10.0 (-16.9) and +/- 5.4 (-10.1) mmHg, respectively. 5. Control values and responses to neck suction at -30 mmHg were compared in subjects grouped in four age bands between 19 and 80 years. With increasing age, the control value of cardiac index (cardiac output divided by calculated body surface area) decreased, systolic and diastolic pressures increased, and the responses of all the measured variables to neck suction decreased.(ABSTRACT TRUNCATED AT 250 WORDS)