1. This is a report of changes in reflex excitability of flexor motoneurons in response to innocuous mechanical stimuli following initiation of an acute experimental inflammation of the knee joint in the chloralose-anaesthetized cat spinalized at level T12. 2. Activity of functionally single alpha- or gamma- motoneurons, identified by ventral root stimulation, was recorded in filaments of the nerve to biceps semitendinosus before and after onset of an acute inflammation. The inflammation was evoked by injection into the knee cavity of the compounds kaolin and carrageenan. 3. Measurements were made of resting activity, responses to local pressure applied to parts of the upper or lower limb, and to flexion or extension movements at the knee joint before and after onset of the inflammation. In two experiments, one in which no inflammation was initiated, and another in which recordings were made only after the inflammation had fully developed, a survey of large numbers of neurons (44 alpha and 84 gamma) showed that under control conditions only 14% of alpha-motoneurons showed a response to mechanical stimulation compared with 41% of gamma-motoneurons. In the presence of an inflammation 41% of alpha-cells were responsive compared with 64% of gamma-cells. 4. Inflammation-induced changes in activity of motoneurons including both excitatory and inhibitory effects, took 1-2 h to fully develop. Excitatory effects included a rise in resting discharge, an increase in the response to local pressure, and an increased response to flexion and/or to extension of the knee. Inhibitory effects included falls in resting discharge and/or in the responses to leg movements. 5. For 35 cells identified as alpha-motoneurons and tested both before and after initiation of an inflammation, 20 remained unresponsive throughout the experiment, whereas 4 that had not responded during the control period began after inflammation to respond to local pressure and/or flexion/extension movements. Of 11 units that exhibited some response before inflammation, 8 showed an increase with inflammation, whereas 2 became unresponsive. 6. The inflammation had rather more dramatic effects on gamma-neurons. Many showed control responses to leg movements, and these were measurably modified by inflammation. Of 56 gamma-cells tested under control conditions, 26 were unresponsive to all stimuli, whereas 30 showed some form of response including activity during flexion and extension movements of the leg. Where responses were tested both before and after onset of an inflammation, 11 of 13 unresponsive units remained unresponsive.(ABSTRACT TRUNCATED AT 400 WORDS)