Bladder stone formation in guinea pigs is a common emergency in exotics practice. Male patients commonly present with hematuria, straining to urinate, pain, acute collapse, or anorexia. In females stones are more likely to be passed or able to be manually removed from the urethra, however surgical emergencies are common in both. On non-emergent exam the vet or owner may find sludge in the urine indicating the patient may be a risk for stone development. Data from the Minnesota Urolith Center from 1981 to 2007 shows 92.6% of guinea pig uroliths submitted were identified as calcium carbonate (878/948) with only 0.4% composed of calcium oxalate (4/948) (1). After diagnosis of bladder stones most owners are counseled about risk of recurrence, and on dietary changes for prevention. Classically owners are told to exercise caution with high calcium vegetables, discontinue pellets, and ensure to compensatorily offer vitamin C supplementation in some form. An evaluation of renal physiology reveals evidence that high nitrate foods including parsley, celery and cucumber skins, combined with excess vitamin C supplementation are likely causative factors in stone formation. Additionally, the role of underlying infectious causes for renal damage such as E cuniculi are likely under-represented as contributing factors. Genetics may play a role in formation, however better prevention can reduce risk of recurrence. This lecture will evaluate the physiology behind urinary calcium excretion to help demonstrate a possible correction in the cause for stone formation in guinea pigs.