Within the intensive care setting, a portable microcomputer system was used to extract three parameters from the intracranial pressure fluctuation associated with the cardiac cycle. One parameter, the mean of sampled intracranial pressure, was defined as the average value of pressure for a 1.08-second interval following the R wave of the electrocardiogram. Another parameter, the amplitude of intracranial pressure, was defined as the difference between the mean and the peak value of the sampled intracranial pressure for the interval considered. The third parameter, a latent interval, was defined as the time period between the occurrence of the R wave and the occurrence of the peak value of the subsequent intracranial pressure fluctuation. Six adults and one pediatric patient were monitored. Both the amplitude and the mean of sampled pressure tended to vary inversely with the latent interval. For the adult patients, the latent interval varied between 503 and 804 ms; the mean pressure ranged between 2.4 and 19.0 mm Hg and the amplitude pressure ranged between 0.6 and 7.2 mm Hg. The latent interval for the child was much shorter (ranging between 269 and 325 ms), and both the mean and the amplitude pressures were much higher (ranging between 38.4 and 57.3 mm Hg and 14.2 and 16.5 mm Hg, respectively). Statistical correlation between hourly pulse rates and the latent interval among the adult cases revealed little association (r = -0.20). For all patients considered, the correlation between the amplitude and the mean of sampled intracranial pressure was quite high, with an r value of +0.91. These reported observations support a conceptual model in which blood volume changes associated with the cardiac cycle occurring within the semirigid craniospinal sac are assumed to underlie the fluctuation of intracranial pressure.