Microdialysis has been used for in vivo studies of extracellular amino acids in rat brain. We describe a method where a probe was designed to be implanted vertically. This probe is suitable for regional stereotaxic studies of the rat brain. The dialysis probe was perfused with Ringer's solution and the perfusates were analysed for their amino acid content with a high performance liquid chromatography (HPLC) technique. An orthophthaldialdehyde derivative of the amino acids was formed before the sample was injected onto the column. In vitro studies of the dialysis probe show that the relative recovery of substances outside the membrane is dependent on perfusion speed and length of dialysing membrane but not on the concentration outside. We were also able to show that the probe was within the blood-brain barrier (BBB) when implanted into the brain since after intravenous injection of Na99mTcO4, a substance that cannot pass through the intact BBB, it was not possible to recover any isotope from the perfusate. We conclude that microdialysis is a unique method of studying regional neurochemical events within the BBB, for example, changes in putative amino acid neurotransmitters and their metabolites.