The cricket Gryllus bimaculatus forms a stable olfactory memory that lasts for practically a lifetime. As a first step to elucidate the cellular mechanisms of olfactory learning and memory retention in crickets, we studied the dependency of memory retention on the de novo brain protein synthesis by injecting the protein synthesis inhibitor cycloheximide (CHX) into the head capsule. Injection of CHX inhibited ³H-leucine incorporation into brain proteins by > 90% for 3 hr. Crickets were trained to associate peppermint odor with water (reward) and vanilla odor with saline solution (non-reward) and were injected with CHX before or at different times after training. Their odor preferences were tested at 2 hr, 1 day and 4 days after training. Memory retention at 2 hr after training was unaffected by CHX injection. However, the level of retention at 1 day and 4 days after training was lowered when CHX was injected 1 hour before training or at 1 hr or 6 hr after training. To study the time course of the development of CHX-sensitive memory phase, crickets that had been injected with CHX at 1 hr after training were tested at different times from 2 to 12 hr after training. The level of retention was unaffected up to 4 hr after training but significantly lowered at 5 hr after training, and the CHX-sensitive memory phase developed gradually during the next several hours. CHX dissociates two phases of olfactory memory in crickets: earlier protein synthesis-independent phase (< 4 hr) and later (> 5 hr) protein synthesis-dependent phase.