The rhythmic, slow electrical activity (RSA) of the hippocampus has been correlated with many diverse processes and behaviors. The specific frequency content of RSA varies widely under various experimental conditions which indicate that the frequency shifts reflect changes in a general arousal continuum. We think that RSA per se is merely one of many consequences of sensory activation of brain stem reticulum (Brain Res. 41: 331-344), but that frequency variations reflect the different information processing modes that cause all of the processes and behaviors with which RSA has been correlated. This idea was tested by 1) evoking reversible RSA by electric stimulation of brain stem reticulum, 2) altering processing modes by simultaneous stimulation of other brain areas, and 3) measuring RSA frequency changes. Test sites (mostly caudate, parietal neocortex, piriform cortex, amygdala, and contralateral hippocampus) were stimulated with 10 and 250 pulses/sec. Test site stimulation at 10/sec usually evoked a 10/sec RSA, which persisted during brain stem stimulation, except when it was frequency modulated in 7 of 26 tests when the test site was piriform cortex or amygdala. With all test sites, high frequency stimulation could modulate RSA evoked from the brain stem. With both low and high frequency stimulation of test sites, some concurrent stimulations caused frequency shifts in a direction opposite from that which would be predicted by an arousal continuum theory.