A combined electrophysiological, behavioral, and biochemical study was initiated to determine the effects of the sulfhydryl-specific reagent fluorescein mercuric acetate (FMA) on olfaction in the tobacco budworm moth Heliothis virescens. The electroantennogram (EAG) response to the standard odorant n-pentyl acetate showed both a time and concentration dependent inhibition by FMA. Treatment of insect antennae with 2.52 × 10−5M FMA for 2 min reduced the EAG by 50%, while treatment for 17 min reduced the EAG by 80%. Incubation of antennae for 7 min with 2.52 × 10−6M FMA resulted in 30% inhibition, while incubation with 2.52 × 10−6M FMA for 7 min resulted in 65% inhibition. Antennal grooming behavior was inhibited by FMA in a similar time and concentration dependent manner as the EAG. Regeneration of previously inhibited behavioral and EAG responses has been observed within a 24-hr period. The interaction of protein, obtained by sonicating intact antennae in phosphate buffer, with FMA was monitored fluorometrically. Successive additions of antennal sonicate to FMA resulted in stepwise decreases in fluorescence. Partial recovery of fluorescence was obtained by addition of cysteine to the FMA-antennal sonicate solution. The polarization of the FMA-antennal sonicate fluorescence decreased upon addition of cysteine. These data indicate that FMA is reacting with a relatively large antennal protein (s) by mercaptide linkage and blocking the olfactory transduction process.