1. Synaptic transmission between olfactory receptor neurons and mitral/tufted cells was examined using a whole-cell recording technique in a hemisected preparation of the turtle olfactory bulb. To determine the olfactory receptor neuron transmitter, we isolated components of the synaptic response of mitral/tufted cells to olfactory nerve stimulation using postsynaptic receptor antagonists. 2. Low-intensity stimulation of the olfactory nerve evoked monosynaptic excitatory postsynaptic potentials in mitral/tufted cells that consisted of a rapid and prolonged depolarization with little contribution from other bulb neurons. The exogenous application of glutamate mimicked the response of mitral/tufted cells to olfactory nerve stimulation. 3. Olfactory nerve stimulation evoked in mitral/tufted cells a two component response that was reversibly blocked by glutamate receptor antagonists. The first, a rapid depolarization of short duration, was sensitive to the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX); the second, a depolarization of slower onset but longer duration, was sensitive to the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP5). When DNQX and AP5 were both present the postsynaptic response was completely abolished. These results strongly support the notion that glutamate is the neurotransmitter at the olfactory nerve to mitral/tufted cell synapse.