Retinal tyrosine level is correlated not only to its serum concentration, but also to the serum tyrosine ratio (the ratio of serum tyrosine to the sum of 5 neutral amino acids, leucine, isoleucine, valine, tryptophan and phenylalanine which compete with it for tissue uptake). In control animals, serum tyrosine ratio, retinal tyrosine, dopamine (DA) and its metabolites, and DOPA synthesis exhibit roughly parallel daily rhythms with night-time lows and peak levels occurring in the early portion of the daily light phase (L + 4 HR). In rats made diabetic by streptozotocin injection serum leucine, isoleucine and valine are elevated, lowering the serum tyrosine ratio and retinal tyrosine level by about 50% throughout the daily cycle. Parameters of DA turnover are still highest in the light phase but normal peak levels are delayed by 4 h until the latter half of the light phase (L + 8 HR). In diabetic rats, the normal increase in retinal DA synthesis and release upon exposure to light may be compromised by the decreased availability of its precursor, tyrosine.