1. We sought to determine if catecholamine biosynthetic enzymes of spontaneously 
hypertensive rats (SHR) differed from those of normotensive Wistar--Kyoto (WKY) 
and Sprague--Dawley (SD) control rats before birth. 2. By immunocytochemical and 
biochemical methods we compared strains for the time of appearance and 
maturation of the enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase 
(DBH) and phenylethanolamine-N-methyltransferase (PNMT) in sympathetic ganglia 
and adrenals. 3. The time of appearance of enzymes was identical in all three 
strains: TH and DBH first appeared in sympathetic ganglia on embryonic day 11 
(E11) and in adrenal medulla on E16. PNMT, restricted to adrenal medulla, 
appeared later on E18. 4. The activity of adrenal TH prenatally on E18 and E21 
and at day of birth (P1) in SHR was approximately two fold that in WKY or SD 
rats. In contrast PNMT was lower in SHR but only on E18. 5. Thus, although the 
timing of the first expression of adrenergic phenotypes is similar in SHR and 
normotensive controls, the differences in TH activity in adrenals suggest an 
enhanced biosynthetic capacity for catecholamines in this strain before birth. 
6. We conclude that SHR differ from normotensive rats from the first expression 
of some of the genes controlling catecholamine biosynthesis.