Bone mineralization was studied in rats. Animals were divided into three feeding groups: LCP - diet with 13.5% crude protein in DM (5% of gluten, 10% of casein), HCP - diet with 21.2% CP in DM (8% of gluten, 10% of casein), and LSM - diet based on grain meals and meat-bone meal (21% CP in DM). After 28 days feeding, animals were euthanased by cervical dislocation and femur bones were collected, weighed and kept frozen until analyses. Diets with 21% protein (HCP, LSM) significantly increased weight of femur bones. Despite of the substantially higher ash level (7.1%) in the LSM diet than in the LCP diet (3.4%), rats of both groups had the similar bone concentration of Ca (15.7 +/- 1.1 vs. 17.4 +/- 1.1 g/kg) and Zn (178.7 +/- 7.9 vs. 173.0 +/- 8.5 mg/kg). However bone density in LSM rats was significantly higher than in LCP ones. Although rats fed HCP diet had intermediate bone density, the bone concentration of Ca (11.4 +/- 0.5 g/kg) and Zn (145.1 +/- 2.9 mg/kg) was significantly lower, than in animals fed LCP and LSM diets. This was related to the very wide protein/calcium (37:1 g/g) and protein/zinc (5.3:1 g/mg) ratios in HCP diet. Those ratios were narrowest in the LSM diet: 16.2:1 (CP/Ca) and 2.6:1 (CP/Zn). It can be conluded that protein/mineral ratio in a diet is a very important factor in bone development, besides dietary protein and ash contents itselves.