Rationale: Phosphodiesterase enzyme type 5 (PDE5) inhibitors and acetylcholinesterase (AChE) inhibitors have cognition-enhancing properties. However, it is not known whether these drug classes affect the same memory processes. Objective: We investigated the memory-enhancing effects of the PDE5 inhibitor sildenafil and AChE inhibitors metrifonate and donepezil in the object recognition task to find out whether acquisition or consolidation processes were affected by these drugs. Methods: The object recognition task measures whether rats remembered an object they have explored in a previous learning trial. All drugs were given orally 30 min before or immediately after learning to study acquisition and consolidation, respectively. Results: Sildenafil given immediately after the first trial improved the memory performance after 24 h and resulted in an inverted U-shaped dose-effect curve with the peak dose at 3 mg/kg. When given before the first trial, sildenafil also improved the memory performance. However, the dose needed for the best performance under this condition was 10 mg/kg, suggesting that the dose-effect curve shifted to the right. This can be explained by the metabolic clearance of the high dose of sildenafil. Donepezil had no memory improving effect when given after the first trial. However, when given before the first trial, a gradually increasing dose-effect curve was found which had its maximum effect at the highest dose tested (1 mg/kg). Likewise, only when metrifonate (30 mg/kg) was given before the first trial did rats show an improved memory performance. Conclusion: Our data strongly suggest that PDE5 inhibitors improve processes of consolidation of object information, whereas AChE inhibitors improve processes of acquisition of object information.