1. Extracellular microelectrode recordings were made from single spinocervical tract (SCT) neurones in the lumbosacral spinal cord of cats anaesthetized with chloralose and paralysed with gallamine triethiodide. 2. Pairs of air-jet stimuli, 60 ms in duration, were used to investigate in-field afferent inhibition in SCT cells. One jet was used to condition the responses to another jet located at a different position within the excitatory receptive field and occurring at times from 100 to 1800 ms later. Fifteen neurones were tested and significant in-field inhibition was observed in all of them. 3. The in-field afferent inhibition was organized spatially in the sense that inhibition was generally strongest when conditioning and testing stimuli were close together and became weaker as they were moved apart. There was also a weak effect due to the strength of the conditioning response; when conditioning produced a strong response, from near the most excitable part of the receptive field, there was often a weak reduction in the test response from distant sites. The inhibitory areas defined in these experiments were generally less than 100 mm in length in units with excitatory receptive fields much longer than this. 4. The in-field afferent inhibition had a time course that lasted from 300 to about 1000 ms. 5. Afferent inhibition was also evoked by applying either air-jet stimuli to hairy skin outside, but close to, the excitatory receptive field or by applying a vibratory stimulus from a piezoelectric transducer (200 Hz) to glabrous skin of the toe pads or the central foot pad. These conditioning stimuli had durations of 20 or 60 ms. For convenience we call this inhibition 'out-of-field' afferent inhibition. 6. Out-of-field afferent inhibition was evoked from both glabrous and hairy skin areas outside the excitatory receptive field. It was common in neurones with receptive fields on the toes and of twenty-eight such neurones tested it was observed in twenty-four. This inhibition had a short latency (usually about 10 ms or less but occasionally up to 30 ms) and lasted for about the duration of the test stimulus (30 or 80 ms when the test stimulus was 20 or 60 ms respectively). It was often followed by a further period of inhibition, with a latency of between 50 and 100 ms and lasting for 60 up to 130 ms. 7. In thirteen SCT neurones more complex effects were seen.(ABSTRACT TRUNCATED AT 400 WORDS)