Rats have developed antipredator defensive adaptations to protect themselves from the large number of animals that prey on them. One such adaptation is the ability to communicate by ultrasonic vocalization, which decreases the likelihood of detection by a predator. Almost all rat vocalizations are inaudible to the human ear as well, so laboratory studies of ultrasonic vocalization require specialized electronic equipment. The most popular of these is the bat detector, which lowers ultrasonic frequency to a humanly audible frequency. Adult rats emit two types of ultrasonic calls: alarm (22 kHz) calls, in aversive and dangerous situations, and appetitive (50 kHz) calls, in appetitive or "friendly" (i.e., nonaggressive) behavioral situations. These two types of calls differ in all acoustic parameters, and their initiation depends on activity in different ascending tegmental pathways to the forebrain: 22 kHz calls require activity in the cholinergic system, and 50 kHz calls in the mesolimbic dopaminergic system. The release of acetylcholine in the diencephalon and forebrain is associated with the emission of 22 kHz vocalizations, and the release of dopamine in the nucleus accumbens with 50 kHz calls. Thus the two calls are reliable predictors of increased cholinergic or dopaminergic activity in the brain. The calls serve as indices of the rat's state, including its affective state, induced by activity of one or the other neurochemical system.