The distribution and the morphology of the mesencephalic neurons projecting to the lumbo-sacral spinal cord levels was investigated with the fluorescent retrograde tracer technique in the cattle. The experiments utilised three male Fresian calves. Fast Blue (FB) was injected at L6-S1 spinal cord levels. After 7 weeks, the animals were sacrificed. The midbrain was removed, fixed, cut on a freezing microtome in serial transverse sections and observed with an epifluorescence microscope. Labelled neurons of the red nucleus, the interstitial nucleus of Cajal, the Edinger-Westphal nucleus, the periaqueductal grey and the mesencephalic reticular formation were counted. Morphometric analysis of labelled neurons was carried out. In the red nucleus we observed about 1500 labelled multipolar neurons, distributed caudo-rostrally along its length (7300 μm). The cells showed a wide area range (430-6280 μm2). FB labelled neurons belonging to the other nuclei were analyzed
