On the basis of electrophysiological experiments performed on turtles (Emys orbicularis, Testudohorsfieldi), the thalamus was subdivided into three zones: lateral, predominantly visual; ventral, predominantly somato-sensory; and medio-central. Morphological experiments with the use of different methods of silver impregnation (Bielschowsky, Nauta, Fink-Heimer) showed that all three thalamic zones received direct afferents from the optic tectum, their density being maximal in the n. rotundus (medio-central zone), moderate in n. GL(v) (lateral zone) and lowest in structures of the ventral zone. Different patterns of evoked potentials (EP) and single unit activity could be recorded in each of these structures after stimulation of the optic tectum. In the lateral zone n. GL adjoining the tr. tecto-thalamicus proved to be a relay station forwarding the short latency impulses from the optic tectum and optic nerve to the forebrain. Later impulses coming from the retina (via optic tectum) and elicited by stimulation of the optic tectum were found to be relayed in the n. rotundus. The ventral zone structures appeared to relay somatic impulses and to exert the predominant influence upon the latest components of flash-evoked and tectally elicited EP in the general cortex. Total bilateral ablation of the optic tectum 1.5–2 months before acute experiments did not abolish functioning of the visual geniculo-cortical and somatic thalamo-cortical systems but produced a dramatic depression of evoked visual activity in the n. rotundus. It is concluded that the influence of the optic tectum on the formation and progressive development of the thalamo-telencephalic systems in the course of the evolution was exerted via two main routes: through the thalamic structures involved in the visual and somatic projection systems to the telencephalon, and through the specialized tecto-thalamic system devoid of direct afferent input (the medio-central zone, mainly the n. rotundus). The latter represents a most important brain system, probably combining modulatory and primitive associative function