Cat alpha-motoneurones supplying muscle units of the soleus S type and gastrocnemius S, FR and FF types were injected intracellularly with horseradish peroxidase. 2. Ten alpha-motoneurones of each motor unit type were collected for morphological analysis of the intramedullary parts of their axonal systems including the recurrent axon collaterals. 3. The alpha-motor axon diameters in the spinal cord white matter were significantly smaller for the soleus motor units (mean = 5.2 micrometer) than for the gastrocnemius S units (mean = 6.4 micrometer), which in turn differed significantly from those of the gastrocnemius FR (mean 7.4 micrometer) and FF (mean = 7.3 micrometer types. 4. The number of axon collateral outbulgings, interpreted as synaptic terminals, which originated from the soleus motor axons (mean = 27.8) was significantly smaller, white that of the gastrocnemius FF motor axons (mean = 98.1) was significantly larger, than the number of outbulgings from the gastrocnemius S (mean = 44.4) and FR (mean = 53.8) motor axons. The latter two types were not significantly different. 5. In transverse sections the axon collateral outbulgings were found not only in the classical Renshaw cell area ventromedial to the main motor nuclei but also within the homonymous motor nucleus. However, only some of the neurones projected to the latter area. Rostrocaudally, the outbulgings were distributed within a distance of less than 1 mm around the position of the parent cell bodies. 6. Some physiological implications of the observed differences between axon collateral systems of various functional types of alpha-motoneurones were discussed in relation to the present knowledge of the effects of the recurrent axon collaterals during different types of involuntary and voluntary activation of the alpha-motoneurones.
