Urinary calculi formation is often observed in guinea pigs (Cavia porcellus), mainly calcium-containing uroliths (calcium carbonate, calcium phosphate, calcium oxalate) are found. Stone formation processes are very common in mammals, however, rodents are more susceptible in comparison with other species. The aim of the trial was to evaluate the impacts of nutrients on stone formation. Density and pH-value of the urine are parameters which highly influence stone generation. In the present study the influence of giving fresh, water containing feedstuffs was tested (salad, carrots, apples), as well as feed mixture, supplemented with methionine in concentrations of 1.75; 2.75; 3.35; 4.95 and 5.75 g/kg was used in this trial. The different ratios were tested on 16 female guinea pigs of 2.5 years old. Adjusting period took 9 days, urine collection period lasted 5 days. Urine volume, density and pH-value were determined daily within the 5 day collection period. For the determination of the mineral content (calcium, magnesium, phosphorus), the urine of the 5-day-period was collected and homogenized. The microscopic evaluation of urines revealed that the most frequently excreted stone types are calcium carbonate, calcium phoshate and struvite. The obtained results suggest, that fresh green feedstuffs increased urine volume and reduced pH values. Methionine premixes had no marked impact on any of the urine parametres observed.