Mitral/tufted cells in the olfactory bulb and projection neurons (PNs) in the insect antennal lobe are involved in complex synaptic interactions with inhibitory interneurons to help shape their odor-evoked responses. In the moth Manduca sexta, both gamma-aminobutyric acid (GABA) and the GABAA receptor agonist muscimol hyperpolarize and lower input resistance in many PNs, often blocking ongoing spike traffic. The GABA response mimics a short-latency, chloride-mediated inhibitory postsynaptic potential (IPSP) evoked in PNs by electrical or odor stimulation of afferent inputs, and the classical GABAA receptor antagonist bicuculline methiodide (BMI) quickly and reversibly blocks this IPSP. Focal injection of BMI (100 microM) immediately preceding a GABA pulse blocks the hyperpolarization evoked by GABA, but a similar injection of BMI preceding an acetylcholine (ACh) pulse fails to block the depolarization evoked by ACh. Moreover, the temporal pattern of odor-evoked activity in moth PNs is also strongly and reversibly altered by BMI. Importantly, the temporal pattern of the response depends on the temporal characteristics of the stimulus: continuous stimulation evokes more complex, rhythmic responses, whereas a pulsatile stimulus can be copied with a discrete burst of spikes for each pulse. Collectively our results indicate that PNs in the moth antennal lobe possess GABA receptors that share certain characteristics in common with vertebrate GABAA receptors. These receptors are largely responsible for helping PNs integrate information about both the molecular features and the timing of olfactory input to the brain.