The membrane potential of olfactory bulb synaptosomal fractions was monitored with the lipophilic cation tetraphenylphosphonium (TPP+), which has been reported to distribute across membranes according to the Nernst equation. The properties of the synaptosomal membrane potential as monitored with TPP+ were similar to those reported for neural tissues using other measurement techniques. There is an electrical potential (delta psi) of -64 and -77 mV in the P1 and P2 synaptosomal fractions, respectively. This potential is due primarily to the K+ diffusion gradient across the synaptosomal membrane. The influence of ouabain on TPP+ accumulation indicates that the (Na+,K+)-ATPase electrogenicity contributes about -20 mV to the resting synaptosomal membrane potential. Veratridine induced a decline in TPP+ accumulation which was blocked by tetrodotoxin or by the omission of Na+ from the medium. A significant mitochondrial contribution to TPP+ accumulation, which varied as a complex function of TPP+ concentration in the medium in a manner indicating that TPP+ interfered with the maintenance of mitochondrial potential, was observed. This mitochondrial contribution could be eliminated by performing the experiments anaerobically in the presence of oligomycin. The results are discussed with relation to the future possible use of TPP+ for delta psi measurements in synaptosomal preparations.