Imbalances in mineral metabolism may cause delayed growth and alter several physiological functions. The aim of this study was to evaluate mineral metabolism in growing calves under organic production (41 males or neutered, 23 females). The diets were based on alfalfa hay plus concentrate supplement, mountain pasture plus concentrate supplement or total mixed ration with organic soybean by-product as main raw material. Blood samples were collected between spring and summer (1 to 4 samples/animal), and weight of calves was determined (291 ± 63 kg). The serum concentration of calcium, phosphorus, magnesium minerals and sodium, chloride and potassium electrolytes were analyzed. The data were analyzed by ANOVA considering sex, diet and fattening season as fixed effects and the weight as a covariate. Calf sex only affected the calcium:phosphorus ratio, which was lower in females than in males (0.93 vs. 1.06, P < 0.05). Also, sodium and potassium levels were higher in females than in males (153.9 and 6.5 vs. 148.6 and 6.0 mmol/L, respectively, P < 0.05). The diet had a significant effect on magnesium concentration, which was lower in the group of animals fed with complete mixed ration than in the rest (0.7 vs. 1.1 mmol/L, P < 0.05). Likewise, the calcium:phosphorus ratio was lower when calves fed hay plus concentrate than when they fed pastures plus concentrate (0.92 vs. 1.06, P < 0.05), while the group fed the complete mixed ration was not different from the rest (P > 0.05). The fattening season affected the blood concentration of calcium and magnesium, with lower values in spring than in summer in both minerals (3.7 and 0.8 vs. 4.8 and 1.1 mmol/L, respectively, P < 0.01). Electrolyte levels were not affected by diet or season (P > 0.05). In conclusion, the growing season and feeding system are factors affecting blood minerals, which may be considered when conducting nutritional deficiency appraisal.