Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational day 12 and 13 (E12 + E13) until the day before parturition (E21 + 22) in order to label all the multiplying precursors of neurons. At 60 days of age the proportion of neurons generated (or no longer labelled) on specific days was determined in the separate layers of the superiorr colliculus. Neurogenesis begins with layers V and IV on day E12; the bulk (87%) of these cells are generated on day E13. This early-produced band of large neurons, the intermediate magnocellular zone, divides the superior colliculus into two cytogenetically distinct regions. In both the deep and the superficial superior colliculus neuron production is relatively protracted. In the deep superior colliculus neuron production peaks on day E15 in lay VII, on day E15 and E16 in lay VI, and on day E16 (the large neurons excluded) in layer V, indicating an inside-out sequence. In the superficial superio coliculus peak production time of both layer I and II is on day E16 but in the latter region neuron production is more prolonged and ends on day E18. One interpretation of these results is that the two pairs of superficial layers are produced in an outside-in sequence. These three cytogenetic subdivisions of the superior colliculus may be correlated with its structural-functional parcellation into an efferent spinotectal, a deep somatomotor and a superficial visual component. A comparison of neurogenesis in different components of the visuomotor and visual pathways of the rat indicates that the motor neurons of the extraocular muscles, the abducens, trochlear and oculomotor nuclei, and neurons of the nucleus of Darkschewitsch are produced first. Next in line are source neurons of efferents to the bulb and the spinal cord: those of the Edinger-Westphal nucleus and the intermediate magnocellular zone of the superior colliculus. These are followed by the relay neurons of the dorsal nucleus of the lateral geniculate body. The neurons of the superficial superior colliculus and of the visual cortex implicated in visual sensori-motor integrations are produced last.