Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, settling pattern, migratory route, and site of origin of neurons of the reticular nuclear complex of the thalamus. On the basis of its chrono-architectonics, the reticular nucleus was divided into a central, medial, and lateral subnucleus. The central subnucleus is the earliest produced component of the entire thalamus with over 50% of its neurons being generated on day E13 and another 40% on day E14. Peak production of neurons of the lateral and medial subnuclei is on day E14. There is a lateral (earlier) to medial (later) neurogenetic gradient between these two components of the reticular complex: only about 12% of the lateral subnucleus neurons, but close to 30% of the medial subnucleus neurons, are generated on day E15. Because the lateral and medial subnuclei display the typical outside-in gradient found in the thalamus, they are considered to constitute a single cytogenetic sector; the early generated central subnucleus, which violates this order, is considered to constitute a separate cytogenetic sector. Observations are presented that neurons of the central reticular subnucleus originate in a unique neuroepithelial region, the reticular protuberance. The migration of heavily labeled cells was traced from this region in rats labeled with 3H-thymidine on day E13 and killed on the subsequent days. The neurons of the lateral and medial reticular subnuclei originate in the reticular lobule of the thalamic neuroepithelium. The migration of heavily labeled, spindle-shaped cells was traced from this region in rats labeled with 3H-thymidine on days E14 and E15 and killed at daily intervals thereafter. The neurogenetic gradient of the reticular thalamic complex seen in postnatal rats is established before birth.