Hyperadrenocorticism is a disease documented in guinea pigs, and is likely under-reported since the clinical signs can overlap with ovarian cysts which are more widely reported as a cause of bilateral flank alopecia. Despite this, the current literature is sparse in terms of documenting reference intervals for the most common tests. There is some information published regarding salivary cortisol levels, but few resources exist for blood references. This prospective study used 21 young healthy guinea pigs to establish reference intervals for several common hyperadrenocorticism tests. These included resting cortisol, adrenocorticotropic hormone stimulation test, low dose dexamethasone suppression test, and urine cortisol:creatinine (UCCR) ratios. The results are being analyzed currently, but preliminary evaluation reveals higher circulating cortisol levels which can render diagnosis challenging. Additionally, UCCR is unreliable for testing in guinea pigs.