Abstract:The effect of prolonged exposure to light on the activity of dopaminergic neurons and dopamine (DA) metabolism of chick retinae was investigated. α‐Fluoromethyldopa, a potent and specific irreversible inactivator of aromatic amino acid decarboxylase, was used to assess DA turnover after inhibition of synthesis, and also to assessin vivotyrosine hydroxylase activity by dihydroxyphenylalanine accumulation. After 48 h of light exposure, retinal DA in 12‐day‐old chicks was about 30% higher (p<0.005) whereas dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were elevated two to three times (p<0.005) the level of controls kept in the dark for the same period. DA turnover was about twofold faster in the light (t1/2= 31 min) than in the dark (t1/2= 65 min). Tyrosine hydroxylase, assayedin vitrowith saturating levels of cofactor and substrate, increased by about 50% after light exposure. The apparent tyrosine hydroxylase activityin vivowas approximately sixfold higher in the light than the dark. These results are interpreted and discussed in terms of the regulation of DA synthesis, and the use of DOPAC and HVA as indices of DA function in the