Since there was no study available on the comparative anatomical neurochemistry of the noradrenaline (NA) and adrenaline (A) containing neurons of the lower brain stem, we studied the distribution of the activities of the three major catecholamines (CA)-synthesizing enzymes in coronal sections of the rat medulla oblongata dissected into microcubes. In the dorso-medial region, there was a 1500 micron rostro-caudal difference in the localization of the peak of PNMT activity compared with the peaks of TH and DBH activities. This result led to a new microdissection technique allowing the preferential microdissection of the C2 A neurons versus the A2 NA neurons. The response of these two populations of CA neurons was then studied after a sustained decrease in blood pressure induced in young SHR by a 14 days dihydralazine treatment. The C2 adrenergic region exhibited an overall increase in TH, DBH and PNMT activity (+69%, +45% and +33%; p less than 0.01 respectively) while the A2 noradrenergic region was unaffected. Thus, the NA and A neurons of the rat dorso-medial lower brain stem do not seem to exhibit the same biochemical response after a prolonged hypotension. This preliminary result favors the hypothesis of a different functional role for the neighboring A2 and C2 neurons in central control of blood pressure.