We present a simple and sensitive assay for tyrosine hydroxylase (TH) using high-performance liquid chromatography (HPLC) with a fluorescence detector which enabled us to study bovine retinal TH of a small quantity in crude retinal samples. The detection limit with this method was 1 pmol of dihydroxyphenylalanine (DOPA) enzymatically produced. Retinal TH exhibited the maximum activity at pH 6 to 6.3, and almost none at physiological pH of above 7, a tendency similar to that reported for striatal TH.