In this study, the plasma glucose concentration and the spectral density signals of the oral CO₂ production (VCO₂) were evaluated. Two groups of individuals were formed: Group I (Control, n = 10) were healthy subjects and Group II (n = 12) were glucose intolerant patients. 24 hours previous to the oral glucose tolerance test (OGTT), several parameters including glucose, insulin, and lipids were measured. The participants were fasted 12 hours and measurements of plasma glucose were carried out during 1 hour, taking blood samples every 10 minutes. Then, a glucose load (75 g) was given. Blood samples were newly taken with the same periodicity to measure glucose and insulin. Simultaneously, the VCO₂ signal was registered. Spectral densities and their respective spectral energies were also obtained every 10 minutes. Statistical significance (p < 0.05) for glucose concentration between Group II and Group 1 was observed. Changes in the spectral density of VCO₂ were observed when the glucose load was given. These changes in spectral density were in accordance to that of glucose and insulin levels after oral glucose load administration during OGTT, however, they did not show statistical significance with the glucose and insulin levels. These results contribute to understand the behavior of the CO₂ produced by the glucose metabolism in glucose intolerant patients during OGTT.