Does an open skull alter the fundamental biomechanical properties of normal brain tissue? This question was studied in 32 anesthetized cats, 16 of which underwent a standard craniectomy (2.5 X 2.0 cm) in the left frontoparietal region. Brain tissue pressure, regional cerebral blood flow (rCBF), and brain water content were measured from the same area of cortical gray and white matter, and intracranial pressure (ICP) was recorded from the cisterna magna. Brain tissue resistance, tissue compliance, and the pressure-volume index were analyzed in response to a bolus injection of saline into brain tissue or the cisterna magna. Cerebrovascular resistance was also calculated. In craniectomized animals 2 hours after surgery, ICP had fallen to 3.75 +/- 0.39 mm Hg, and cortical gray and white matter tissue pressure had fallen to 3.19 +/- 0.47 and 4.69 +/- 0.54 mm Hg, respectively (mean +/- standard error of the mean); these variables did not fall further over 4 hours. The pressure-volume index in the same animals increased significantly from 0.67 +/- 0.01 to 0.86 +/- 0.04 ml. Tissue compliance rose in the cortical gray matter but tissue resistance fell, approximating that found in subjacent white matter. There was no significant difference between animals with and without craniectomy in rCBF, cerebrovascular resistance, or brain water content in either gray or white matter. These findings indicate that in the cat craniectomy causes an increase in the compensatory capacity of the intracranial cavity to increased volume. The data also indicate that cortical tissue has high hydraulic conductivity and compliance when the skull is opened.