The effects of i.p. amphetamine administration (5 mg/kg) on the evoked unitary responses of substantia nigra (SN) neurons to electrical stimulation of their afferents were tested in 4 kittens (3-27 days of age) and 4 adult cats. In adults, amphetamine had two major effects: (1) it blocked temporarily (15-30 min) all neuronal responses caudate (Cd) and cortical (Cx) stimulation; neuronal responsiveness recovered by 75 min post-drug; and (2) after 15 min postdrug, Cd and Cx stimulation evoked initial excitatory responses that were almost never found predrug. The latencies of Cd-evoked excitations indicated the existence of a mono- or oligosynaptic excitatory strionigral pathway while latencies of Cx-evoked excitations suggested that corticonigral excitatory influences were mediated multisynaptically. In kittens, amphetamine also produced an initial blockade of Cd- and Cx-evoked responses. However, the sign of initial responses to Cd stimulation was not altered since excitations were found both before and after drug treatment. These results indicated that amphetamine reveals excitatory evoked responses of SN neurons to striatal and striatally-mediated inputs that are masked during the course of normal postnatal development. Drug-related alterations of afferent inputs to SN neurons may underlie amphetamine-induced shifts in spontaneous neuronal activity which have been reported frequently.