1 The characteristics of the group Ia synaptic input to triceps surae motoneurones have been examined in pentobarbitone-anaesthetized cats, using intracellular recording techniques. The mechanical properties of the muscle units innervated by the cells were determined and the motoneurone input resistance values were also measured.2. A significant positive correlation was found between the maximum amplitudes of homonymous composite (electrically evoked) monosynaptic excitatory post-synaptic potentials (EPSPs) and the motoneurone input resistance values across the entire population of units sampled. The same correlation in the case of heteronymous EPSPs was also significant although somewhat less strong.3. The distribution of the amplitudes of unitary miniature EPSPs (mEPSPs) of presumed group Ia origin, elicited by small static stretches of the homonymous muscles, were also studied. A significant positive correlation was found between the median amplitudes of the mEPSP distributions and the input resistance values in the motoneurones studied. Positive correlation was also observed between the amplitudes of the median mEPSPs and the maximum homonymous composite EPSPs in the cells for which both data points were available.4. In each of these correlations, the synaptic potential amplitudes tended to be larger in the relatively high resistance type S (slow twitch muscle unit) motoneurones from both gastrocnemius and soleus motor pools, than in the lower resistance type F (fast twitch muscle unit) gastrocnemius cells.5. Examination of the shape of the homonymous monosynaptic EPSP wave forms in different motoneurones showed that these tended to be significantly longer in duration in type S cells than in type F. This difference could not be entirely accounted for by the relatively small difference in mean time constant values found in types F and S cells.6. The results suggest that the density of group Ia synaptic terminals tends to be higher on type S motoneurones than on the type F cells. Further, cells receiving a relatively high density of group Ia input apparently tend to have a greater proportion of this input distributed to distal membrane regions than is the case in motoneurones receiving a relatively low input density.