Due to the high incidence of hypomagnesemia in some countries and the absence of studies in Brazil, we want to evaluate the concentrations of magnesium in the vitreous humor of slaughtered bovines in the state of Rio de Janeiro in early spring, aiming to investigate the occurrence of animals under conditions of clinical hypomagnesemia. A total of 116 eyes of adult bovines of different ages and breeds were evaluated. We extracted an eye of each animal to evaluate, immediately after the slaughter and were manipulated to obtain the vitreous humor, under appropriate temperatures conditions. The vitreous humor was collected by aspiration with a 27x7 mm needle, inserted in the lateral corner of the eye. Approximately 1.0 ml of liquid was centrifuged (3000rpm, 15 minutes), the supernatant aspirated with a disposable micropipette, packaged in eppendorfs tubs without preservatives was kept at -20 °C. Tests were performed to determine magnesium concentrations using commercial kits in spectrophotometer automatic digital. The mean of the values obtained (1.99±0.3 mg/dL) was normal or low according to the source consulted. In 66 animals (56,9%) the concentration of magnesium in the vitreous humor was lower, 2.02 mg/dL, indicative of hypomagnesemia, demonstrating a high probability condition of clinical hypomagnesemia (tetany) can develop in the conditions outlined in the present study. The possibility of making a post-mortem diagnosis through vitreous humor analysis is important for the prevention of these diseases and to search for curative measures in those risky areas.