1. We have investigated whether pulmonary stretch receptors are the only lung receptors determining the time of onset of inspiratory efforts in anaesthetized, paralysed rabbits. 2. New Zealand White rabbits were anaesthetized, paralysed and ventilated by intermittent positive pressure with a pattern that closely followed spontaneous breathing. Inspiratory efforts were recorded as bursts of activity in a root of the phrenic nerve. Lung stretch receptors were blocked with SO2 in air. Abolition of the Breuer-Hering reflex was used as an index of stretch receptor block. 3. With stretch receptors functioning phrenic discharge invariably occurred during the deflation phase of ventilation. With stretch receptors blocked phrenic discharge occurred with no set relation to ventilation at spontaneous resting tidal volume but was locked to inflation and deflation phases of lung volume at 30% higher tidal volumes. 4. Bilateral vagotomy produced a pattern of phrenic discharge identical with that seen with stretch receptor block and low ventilating volumes. 5. Thus we have demonstrated a vagally mediated inspiratory initiating effect; it probably originates from rapidly adapting lung irritant receptors.