Interstitial fluid pressure (IFP) in the rat brain was measured with micropipettes, 2-4 .mu.m in diameter, after trepanation and incision of dura. IFP was compared with CSF pressure (CSFP) measured by a 0.3-mm (OD) cannula in the lateral ventricle. Control IFP averaged 3.43 .+-. 0.65 (SD) mm Hg (n = 83) with a corresponding CSFP of 3.53 .+-. 0.71 (n = 83). Pretreatment with indomethacin to reduce inflammation during preparation did not affect IFP and CSFP. Infusion of a 20% mannitol solution amounting to 5% of body weight resulted in an average IFP of -5.66 mm Hg and CSFP of -4.56 mm Hg 20 min after start of the infusion, after which a gradual rise in both pressures was observed. Similar results were obtained after infusion of hypertonic urea. Inflation of a cuff around the rat neck resulted in an immediate and similar rise in IFP and CSFP. Increasing the arterial CO2 partial pressure to 80 mm Hg increased IFP to 12-14 mm Hg and pial venular pressure to 13-15 mm Hg, while CSFP rose to about 10 mm Hg. The micropuncture technique for measurement of brain IFP is rapid and practically atraumatic and may be used in a variety of animals.