1. The major protein synthesized and secreted by the choroid plexus from mammals, birds, reptiles and probably amphibians is similar in subunit structure to transthyretin. 2. In mammals and birds the proportion of transthyretin mRNA is much higher in choroid plexus RNA than in liver RNA. No transthyretin mRNA is found in brain outside the choroid plexus. 3. Transthyretin-like protein, such as that secreted by the choroid plexus, was not detected in amphibian serum and was present in very low levels in reptile serum. 4. It is proposed that transthyretin synthesis and secretion arose earlier in evolution in the choroid plexus than in the liver.