Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses and young pups were analyzed in order to delineate the boundaries of the proliferative thalamic neuroepithelium, describe its early transformations, identify its regional divisions, and, finally, attempt to relate its distinct neuroepithelial components to specific thalamic nuclei that they supply with neurons. On day E13 the thalamic neuroepithelium consists of two divisions, the rostral lobe and the caudal lobe, and interposed between the two is a small transient structure, the reticular protuberance. By day E14 the rostral lobe has become partitioned into the anterior lobule and the reticular lobule, and the caudal lobe into the intermediate lobule and the posterior lobule. By day E15 these four lobules have become further partitioned into sublobules, characterized as regional eversions and inversions (concavities and convexities) of the thalamic neuroepithelium. Several of these sublobules are still recognizable on day E16 but progressively disappear thereafter. In this introductory paper, some evidence is presented in support of the hypothesis that the identified thalamic sublobules represent putative cell lines committed to produce neurons for specific, early-generated thalamic nuclei. Detailed documentation of the evidence on which the identifications are based is provided in subsequent papers of this series which deal with the early development of specific thalamic regions and nuclei. In our attempt to identify these putative cell lines, we sought to meet the following criteria: (1) a good match between the time course of mitotic activity in a neuroepithelial sublobule and the birth days of neurons in the nucleus that it is postulated to supply with neurons, (2) relative proximity between the putative neuroepithelial source and the thalamic target structure, and, where possible, (3) the tracing of migrating cells from the germinal source to its destination. Using these criteria we have made the following tentative identifications. The early derivatives of the anterior thalamic lobules are the sublobules (committed cell lines) of the anterior thalamic nuclei, and of the central lateral and mediodorsal nuclei. The early derivatives of the reticular lobule and reticular protuberance are the sublobules of the reticular nuclear complex. The early derivatives of the intermediate lobule are the sublobules of the ventrolateral and ventrobasal nuclei. Finally, the early derivatives of the posterior lobule are the sublobules of the dorsal geniculate, ventral geniculate, and medial geniculate nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)