Type I sensory fibers in cat hairy skin innervate structures characterized by 20-50 specialized epithelial (Merkel) cells aggregated in a small dome shaped elevation. One type I fiber approaches each dome and ramifies to supply at least one terminal to each Merkel cell. Mechanical deformation of a whole dome normally produces a continuous, but irregular, discharge. This discharge often contains short (5-10 ms) intervals, even at low (<10 impulses/s) mean frequencies. More regular discharge could be evoked from approximately 50% of the domes examined if a small diameter stimulator was used to deform a restricted area on the dome. Even though the discharge might be largely periodic, shorter intervals were typically interspersed. The periodic discharge was reset by orthodromic impulses occurring at various times after a preceding impulse and by an antidromic impulse injected via the parent axon. After an orthodromic impulse is generated or an antidromic impulse enters the receptor, the excitability of the receptor, as tested with a short mechanical pulse delivered to the whole dome, is depressed for 50-70 ms. At intervals of 5-10 ms the depression is at least 2-3 times threshold. Since a stationary, near threshold stimulus can produce intervals sufficiently short that the excitability of the receptor is appreciably depressed by the first impulse of the pair, some large amplitude excitatory event can occur within the receptor. It is argued that the most likely source of such an event, consistent with other features of the discharge pattern and the morphology of the receptor, is transmitter release by the Merkel cells.