Investigations were conducted on the savanna grasslands area of the Eastern Plains of Colombia to determine (1) the mineral status of the native pastures (Trachypogons and Andropogans) and (2) the effect of mineral supplementation on four herds of 30 to 3 5 grazing Zebu cows. Two herds were supplemented with salt (NaCl) only, and two herds were supplemented with a mineral mixture consisting of NaCl, dicalcium phosphate and trace minerals (Fe, Cu, Co, Zn, Mn and I). One herd on the mineral mixture and one on the salt treatment received a urea-molasses-S supplement during the dry season. This treatment had only a minimal effect. Hand-plucked pasture samples indicated that after the pasture was burned N, P, Na, Mn and Co (P<.01) and Cu and K (P<.05) levels were higher in the dry season than in the rainy seasons. Ca and Mg levels in pasture samples were higher (P<.01) in the early rainy season, while Zn level was lowest (P<.01) in the late rainy season. Levels of Mo and in vitro organic matter digestibility of pasture samples were not influenced by season. Supplementation with the mineral mixture increased live weight of cows (302 vs 330 kg; P<.01) and calf weaning weight (132 vs 172 kg; P<.01) and tended to increase calf weaning percentage (36.5 vs 57.8%). The mineral mixture also increased (P<.01) serum inorganic P of cows, with the degree of the increase depending on the season. In the rainy season, cows receiving the NaCl supplement had serum inorganic P levels lower than 4.5 mg/100 ml and had lower (P<.05) serum Cu levels (82.4 vs 99.8 µg/100 ml) than cows receiving the mineral mixture. In the rainy season, young bulls that received the NaCl supplement showed lower (P<.05) metacarpal density (1.49 vs 1.61), bone ash (62.2 vs 64.5%) and P concentration of bone ash (17.1 vs 17.8%) than bulls that received the mineral mixture. The mineral mixture increased (P<.05) liver Co (.17 vs .34 ppm, dry matter basis) and tended to increase liver Cu (231 vs 342 ppm), but decreased (P<.05) liver Mo (4.45 vs 3.89 ppm) when compared to the salt supplement. It appeared that the amounts of P and Na and, in some instances, Ca and Cu were insufficient for optimum beef production in the Colombian Eastern Plains area.