Activation of the striatal D2 receptor subfamily in rats results in a behavioural syndrome known as stereotypy, made up of repetitive sniffing and gnawing, accompanied by an increase in the animals' activity. The repetitive behaviours observed in people following amphetamine ingestion may have a similar neurochemical basis. By contrast, blockade of the striatal D2 receptor subfamily produces marked increases in muscle rigidity in rats and a Parkinson-like syndrome in humans. In both rats and humans, administration of a D2 antagonist results in a rapid and large increase in prolactin release from the anterior pituitary, as dopamine's inhibition of prolactin release is blocked.