Two "in vivo" techniques allow the monitoring of extracellular levels of monoamines and related compounds in selected rat brainstem regions: voltammetry and microdialysis. "In vivo" voltammetry has a high regional selectivity: for example, we have been able to perform a subregional study and to show that the increase in extracellular DOPAC induced by 30 min-hypotension was twice as larg in the rostral as in the caudal rat locus coeruleus. The spatial resolution, as expressed by x 1/2 (see text), is 4 times better for voltammetry (50 microns) than for microdialysis (190 microns). Another advantage of voltammetry is its excellent time resolution. However, microdialysis has a much better biochemical specificity than voltammetry. Furthermore it allows some enzymatic activities, such as tyrosine hydroxylase, to be measured almost continuously in catecholaminergic brain nuclei. From a functional point of view, the results of our experiments (alpha 2 ligand administration, arterial hypotension or stress) illustrate the respective interest and complementarity of these two "in vivo" techniques. Their current developments will lead to a better temporal and biochemical resolution combined with an increase in the number of substances analyzed "in vivo", including peptides and nitric oxide.