Changes in the firing probability of single motor units in response to electrical stimulation of muscle nerves were used to derive the projections of muscle spindle Ia afferents to the motoneurones of various leg and thigh muscles. Discharges of units in soleus, gastrocnemius medialis, peroneus brevis, tibialis anterior, quadriceps, biceps femoris and semitendinosus were investigated after stimulation of inferior soleus, gastrocnemius medialis, superficial peroneal, deep peroneal and femoral nerves. Homonymous facilitation, occurring at the same latency as the H reflex and therefore attributed to monosynaptic Ia EPSPs, was found in virtually all the sampled units. In many motor nuclei an early facilitation was also evoked by heteronymous low-threshold afferents. The heteronymous facilitation was considered to be mediated through a monosynaptic pathway when the difference between the central latencies of heteronymous and homonymous peaks was not more than 0.2 ms. The heteronymous Ia connections were widely distributed. In particular, monosynaptic coupling between muscles operating at different joints appears to be the rule in humans, though it is rare between ankle and knee muscles in the cat and the baboon.