The respiratory effects on heart rate (HR), right ventricular stroke volume (SV RV) and the pulmonary arterial flow (PAF) have been studied beat-by-beat in seven conscious, healthy subjects. Tidal volumes (VT) of 1.0 and 2.0 liters at a breathing rate of 6 c x min-1 were used as controls. With identical VT and respiratory rate intrathoracic was affected by (A) negative inspiratory pressure (NIP) of 5 cm of water and (B) with intermittent positive pressure ventilation (IPPV). In the control experiments all subjects exhibited an inspiratory increase in HR (respiratory sinus arrhythmia) as well as in SV RV. With VT of 1.0 liter, NIP augmented the inspiratory increase in SV RV and the respiratory oscillations of PAF. IPPV decreased SV RV and HR during inspiration. It is concluded that NIP and IPPV affect the SV RV and the right heart blood flow. The similar effects on HR and SV RV suggest a causal relationship which might be explained by heart rate regulating reflexes from the heart.