The proto-oncogene c-erbA and its analogues are genes that encode thyroid hormone receptors. In rats, at least two loci have been identified by their homology to c-erbA. Each locus can produce at least two distinct mRNA species via RNA processing. However, one of those transcripts, r-erbAα-2, does not yield a triiodothyronine (T3)- binding protein when translated invitro. Proper development of the rat brain is thyroid hormone-dependent during the perinatal period and there is a documented increase in brain nuclear T3-binding capacity during that time. By hybridizing cDNA probes specific to various rat erbA-like transcripts with RNA extracted from developing rat brain, we sought changes in thyroid hormone receptor mRNA levels that correspond with changes in T3-binding capacity in rat brain during the perinatal period. Three mRNA of the erbA family showed variations in relative abundance during brain development. Oddly, r-erbAα-2, a variant that does not code for a T3-binding protein, was the most abundant erbA-like RNA to correlate with the reported changes in T3-binding capacity. The message for rat rerbAα-1 that encodes a functional T3 receptor also varies with T3-binding capacity but at a level that is at least 8- fold lower than the r-erbAα-2 message. The level of rat erbAβ-l message also varies in rat brain during the perinatal period but not in a way that correlates with changes in T3-binding capacity.