This study evaluated the changes in pathways of cerebrospinal fluid (CSF) outflow that accompanied acute and compensated hydrocephalus in the rabbit. Intraventricularly injected 99mTc antimony sulfide was used as a tracer of outflow pathways, and specified structures were counted 12 to 24 hours after injection. Fifteen rabbits were divided into three groups: 1) an acutely hydrocephalic group in which 3 cisternal injections of blood were followed by a study of CSF pressure, ventricular size, and CSF outflow pathways 1 week after the last injection; 2) a control group treated according to the same protocol, except that sterile saline was injected instead of blood; and 3) a chronic group also treated according to the same protocol but in which the animals were maintained an average of 4 weeks after the last blood injection. Ventricular size was measured by computed digitation and expressed as an area ratio of ventricle to brain (VBR). In control animals, 11.8% of the injected colloid dosage was found in cranial perineural lymphatic channels, and 4.8% appeared in the spinal cord. The mean CSF pressure was 149 +/- 20.2 mm H20 (mean +/- SE) and the mean VBR was 0.040 +/- 0.003. In animals evaluated 1 week after subarachnoid injection, accessory cranial perineural lymphatic outflow decreased significantly to 3.4%, and spinal cord activity increased to 9.8% (P less than 0.05, two-tailed t-test). These animals were hydrocephalic and had CSF pressure of 247 +/- 25.1 mm H20 (mean +/- SE) and VBR of 0.083 +/- 0.009.(ABSTRACT TRUNCATED AT 250 WORDS)