1. The use of a direct current (d.c.) to produce a differential block of conduction in the cervical vagus nerves of rabbits is described; the myelinated fibres are blocked, while the non-myelinated ;C' fibres conduct normally. The method produces reproducible and reversible results.2. The block is equally effective for low and high frequencies of discharge (1-100 Hz). During recovery or development of the block, lower frequencies of discharge can pass but higher frequencies cannot.3. Block of conduction in myelinated fibres is associated with slower, deeper breathing, confirming previous work with cooling.4. A further slowing and deepening of breathing may occur when a differentially blocked (;non-myelinated') nerve is sectioned, and this is mainly apparent when there are pathological conditions in the lungs.5. The respiratory response to the right atrial injection of phenyl diguanide is mediated by non-myelinated thoracic vagal afferent fibres.6. The tachypnoeic response to lung deflation is not mediated by non-myelinated fibres.7. Head's Paradoxical reflex has been demonstrated during partial recovery of conduction in myelinated fibres when only lower frequencies of afferent discharge can pass the area of block.8. A standard technique for providing a reproducible vagally mediated, tachypnoeic response to pulmonary micro-embolism is described using inert carbon-coated microspheres of 50 mum diameter. This tachypnoeic response was unchanged during a differential block indicating that the response was mediated by non-myelinated ;C' fibres.9. Pathological changes such as haemorrhage, oedema, infarction and collapse were absent after micro-embolism, and there were no systematic changes in lung resistance and compliance. The walls of arterioles and adjacent alveoli are distorted by the emboli and these areas are the probable sites of afferent stimulation.