1. Activation of pulmonary C fibres (J receptors) in animals produces inhibition of spinal motoneurones. Intravenous bolus injections of lobeline are believed to activate pulmonary C fibres (J receptors) in human subjects and to produce characteristic sensations and cardiorespiratory responses. This study quantified the respiratory sensations evoked by such injections and then used a range of suprathreshold doses of lobeline and tested for the presence of reflex or descending inhibition of motoneuronal output. 2. Injections of lobeline produced dose-dependent sensations of respiratory discomfort referred to the throat and upper chest beginning within about 10 s and often associated with coughing. As the dose increased the latency for the sensations decreased while their duration and intensity increased. Reflex changes in blood pressure, heart rate and ventilation also occurred. 3. Injections of lobeline at doses sufficient to evoke respiratory discomfort lasting 25-32 s (37-73 microgram kg-1) increased the size of the H reflex in soleus with an onset latency of about 10 s and lasting about 20 s. 4. The size of EMG responses evoked in upper limb muscles by transcranial magnetic stimulation of the motor cortex increased shortly after injections and remained elevated for about 30-35 s. 5. Injections of lobeline during sustained voluntary contractions of the elbow flexors at submaximal or maximal levels did not impair the ability to produce force. 6. Walking was not disrupted by repeated suprathreshold doses of lobeline. 7. It is concluded that injections of lobeline sufficient to evoke cardiorespiratory reflexes and sensations of severe respiratory discomfort are not associated with functionally important inhibition of motor performance. The results cast doubt on the ability of the J reflex to limit exercise in humans.