Interstitial fluid pressure (IFP) in the rat brain has been measured with micropipettes, 2-4 microns in diameter, after trepanation and incision of dura. IFP was compared with cerebrospinal fluid pressure (CSFP) measured by a 0.3-mm (OD) cannula in the lateral ventricle. Control IFP averaged 3.43 +/- 0.65 (SD) mmHg (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 mmHg and CSFP of -4.56 mmHg 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 mmHg increased IFP to 12-14 mmHg and pial venular pressure to 13-15 mmHg, while CSFP rose to about 10 mmHg. The micropuncture technique for measurement of brain IFP is rapid and practically atraumatic and may be used in a variety of animals.