1. Antidromic activation of striatal spiny projection neurons by substantia nigra stimulation in vivo did not evoke inhibitory postsynaptic potentials (IPSPs) in the antidromically activated neurons, or in neighboring spiny neurons. 2. More generalized activation of projection cells by stimulation of the efferent pathway in slices did not evoke IPSPs in spiny neurons. Inhibitory mechanisms were operative in these slices, as indicated by the presence of an IPSP component in the orthodromic response to local stimulation. 3. Dual intracellular recordings, obtained from cells located within 50-200 microns of each other in striatal slices also failed to demonstrate any inhibition among striatal spiny cells. Spikes triggered in one spiny neuron by current injection failed to produce any postsynaptic potential in nearby spiny cells at resting or depolarized membrane potentials. Excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of the cortex were not affected by spiking of neighboring cells. 4. It is impossible to rule out the presence of inhibition among striatal spiny neurons in all circumstances. However, the absence of demonstrable IPSPs in these experiments argue against the common view that mutual inhibition among spiny neurons is a central organizing principle of striatal function.