Introduction: Environmental enrichment has gained increasing relevance in the last decades to improve animal wellbeing which is particularly relevant in laboratory animals. Nonetheless while mandatory in European legislation, classical laboratory animals care and maintenance protocols do not include enrichment. Hence, our goal was to review the environmental enrichment protocols currently in rats and mice.

Methods: Five databases (PubMed, Science Direct, Scopus, Web of Science and EBSCO) were searched using the keywords "environmental enrichment" AND (pain OR nociception) AND (rat OR rats OR mouse OR mice) from inception to 30 th of August 2024. The quality assessment of studies was performed using the ARRIVE guidelines. Data management and analysis was performed in Microsoft Excel.

Results: Overall the quality of the studies was considered moderate. The environmental enrichment protocols implemented varied significantly between rats and mice, not only in terms of total number of enrichment items but also in relation to the type of enrichment used for each species. Importantly, a great number of studies stated performing environmental enrichment but failed to report the type of enrichment used.

Conclusions: Despite the importance of implementing environmental enrichment in pre-clinical studies being recognized, there is a significant variation in protocols between laboratories, highlighting the need for standardization.