The present study compared the activities of some of the monoamine synthesizing enzymes in several brain regions, the retina as well as adrenal gland of albino Sprague-Dawley (SD) and Long-Evans hooded (LE) rats. Brainstem, hypothalamic and retinal tyrosine hydroxylase (TH) activity were significantly higher in LE than in SD. In addition to higher enzyme activity, a larger number of TH-immunoreactive perikarya as well as a higher concentration of TH-immunoreactive processes were observed in the retina of LE rats. There was no strain difference in TH activity of caudate nucleus (CN) and substantia nigra (SN). In contrast to brain regions and retina, adrenal TH activity was markedly higher in SD than in LE animals. Aromatic L-amino acid decarboxylase (AADC) activity of both the brainstem and adrenal gland in the LE strain was lower than in SD animals. No differences in the AADC activity of hypothalamus, SN and CN were found between LE and SD strains. Phenylethanolamine N-methyltransferase (PNMT) activity of the hypothalamus, retina and adrenal gland of LE strains was significantly lower than in SD rats. In spite of the difference in the enzyme activity, there were no marked morphological changes observed in PNMT-immunostaining patterns between the retina of LE and SD rats. Tryptophan hydroxylase activity of both the brainstem and hypothalamus did not exhibit strain differences.(ABSTRACT TRUNCATED AT 250 WORDS)