Systems analysis of cerebrospinal fluid pulse wave forms (CSFPWs) was carried out in 19 cats during the inhalation of 5% CO2 + 95% O2. 10% CO2 + 90% O2 and 10% O2 + 90% N2. The results were compared to CSFPWs obtained during the inhalation of 100% O2 and during an intraventricular infusion to the same level of cerebrospinal fluid (CSF) pressure (CSFP) as produced by the test gas. The systemic arterial pressure pulse was utilized as the system input, and the CSFP pulse was used as the output. The harmonic amplitudes of the two pulses and the amplitude transfer function (XFRa) between the pulses were calculated. Hypercapnia and and hypoxia produced an increase in CSF pulse pressure (delta Pcsf), an increase in primarily the XFRa of the fundamental frequency, and as a result, an increase in amplitude of the fundamental frequency of the CSFPW with rounding of the pulse wave. The changes are greater than those noted during an intraventricular infusion (IVI) to the same level of CSFP. In addition, the volume-pressure test was performed on the hypercapnic animals. The volume-pressure response was less during hypercapnia than during the IVI at the same mean level of CSFP. The result suggest that the increase in deltaP csf is related to cerebral arteriolar vasodilation and not to a steepening of the volume-pressure curve and that rounding of the CSFPW is related to a decrease in cerebrovascular tone.