The administration of 8-OH-DPAT to rats produced a dose-dependent suppression of spontaneous locomotor activity in an open field arena. 8-OH-DPAT was administered in the dose range 12.5-1,600 micrograms.kg-1 SC. Vertical activity ("rearing") was more sensitive to the treatment than horizontal activity ("locomotion"), both in terms of potency and efficacy. The activity along the walls of the open field arena ("peripheral activity") was increased, and the rearing activity was decreased, relative to total horizontal activity and total activity, respectively. There were no effects by 8-OH-DPAT on treadmill locomotion. The rectal temperature was decreased by 8-OH-DPAT administration, not only in animals tested in the open field, but also in animals with an increased body temperature, produced by treadmill locomotion.