THE precise role of prostaglandin-D-synthase β (trace protein), the major constituent of cerebrospinal fluid, is unclear. In the present study, a sensitive and highly specific fluoroimmunoassay was developed. The measurement of the enzyme levels in rat CSF revealed a developmental change in the CSF levels with the highest value of 6 6 ± 8μg/ml at 7 days after birth. No significant difference in the levels was seen between different times of day. Subcutaneous injections of all-trans retinoic acid caused a dramatic decrease in the protein levels in a dose- and time-dependent manner. These findings may raise the possibility that prostaglandin-D-synthase in CSF is involved in retinoic acid action on the brain.