Activity within the cat paraventricular hypothalamus (PVH) during sleep and waking states was measured by quantifying intrinsic tissue reflectivity. A fiber optic probe consisting of a 1.0 mm coherent image conduit, surrounded by plastic fibers which conducted 660 nm source light, was attached to a charge-coupled device camera, and positioned over the PVH in five cats. Electrodes for assessing state variables, including electroencephalographic activity, eye movement, and somatic muscle tone were also placed. After surgical recovery, reflected light intensity was measured continuously at 2.5 Hz during spontaneously varying sleep/waking states. Sequential state transitions from active waking to quiet waking, quiet sleep and active sleep were accompanied by progressively increased levels of PVH activity. Overall activity was highest during active sleep, and decreased markedly upon awakening. Moment-to-moment activity oscillated in the 0-0.1 Hz range, especially during active sleep and active waking; this oscillation diminished during quiet sleep. Distinct sub-regions of enhanced or diminished activity emerged within the imaged area in a state-dependent manner. We conclude that PVH activity changes with behavioral state in a regionally specific manner, and that overall activity increases during quiet sleep, and is even more enhanced in active sleep. PVH activation could be expected to stimulate pituitary release of adrenocorticotropic hormone (ACTH) and affect input to autonomic regulatory sites. Since ACTH and corticotropin releasing factor elicit arousal, and since the PVH projects to other brain areas which modulate state, we speculate that the PVH plays a role in shaping characteristics of sleep/waking states.