The reversible transcription inhibitor 5,6-dichloro-1-beta-D-ribobenzimidazole (DRB) was used to examine the contribution of mRNA synthesis to long-term enhancement (LTE) following one-trial conditioning of Hermissenda. Inhibition of mRNA synthesis by DRB or inhibition of protein synthesis by anisomycin did not significantly affect the induction and maintenance of short-term enhancement (STE) examined 1 h after one-trial conditioning. In contrast to the absence of an effect of the inhibitors on STE, LTE was blocked by DRB or anisomycin applied shortly before and during the presentation of the conditioning trial. Consistent with previous reports, animals that received an unpaired CS and US did not exhibit LTE. In addition, a control group that received a concentration of DRB (10(-7) M) that does not significantly affect mRNA synthesis exhibited typical LTE when tested 24 h postconditioning. These results demonstrate that the induction of LTE produced by one-trial conditioning is dependent upon transcription and the regulation of gene expression.