Mice were rendered dependent on morphine by the subcutaneous pellet implantation technique. After three days, the pellet was removed and six hours later, withdrawal jumping was precipitated by subcutaneous naloxone. Brain dopamine (DA), norepinephrine and serotonin were measured at various intervals after naloxone. A sudden increase in DA was noted to occur in dependent mice that jumped but not in those that failed to jump. The increase in DA after naloxone occurred within 2 minutes and was maximum between 5 to 10 minutes when the incidence of the precipitated morphine withdrawal jumping response was at a peak. Brain norepinephrine and serotonin levels remained unaltered after the naloxone challenge in both morphine-dependent and naive groups of animals. The increase in brain DA was greatest in the cortical-striatal area of mice. Similar regional brain studies in the morphine-dependent rat indicated that the greatest elevation in DA occurred in the corpus striatum. Administration of d -tubocurarine to block the jumping response did not prevent the increase in DA. Physostigmine antagonized the naloxone-induced jumping response and prevented the DA increase. The steady-state levels of brain amines at the time of naloxone administration did not appear to be crucial for mediating the jumping response since lowering DA by 60% or elevating brain amine levels by 42% in morphine-dependent mice, respectively, with reserpine or pargyline had little effect on both the naloxone-induced jumping response and the increase in brain DA. Thus, the naloxone-precipitated withdrawal jumping response in morphine-dependent animals appears to be associated with an elevation of brain dopamine level and subject to cholinergic regulation as well.