Penetrating arterioles of the cat brain were examined for nerve fibers by means of electron microscopy. Of 86 arterioles, 57 had axon bundles, in most cases two. The axon bundles consisted of 2 to 12 individual axons, always confined to the adventitial space and separated from the muscle cells by a cleft of at least 80 nm. They were classified into three groups, axons without vesicles, those with few vesicles, and those with many vesicles. The last group must be regarded as functional synapses of the autonomic nervous system. Thus the innervation of arterioles in the cat brain is substantially the same as that in other organs of other animals.