The relationship between the plasma magnesium (Mg) concentration and steady-state plasma insulin (SSPI) and glucose (SSPG) concentrations at the end of a 180-minute infusion of octreotide, insulin, and glucose was determined in 98 healthy nondiabetic subjects. For the purposes of data analysis, the population was divided into tertiles on the basis of the plasma Mg concentration: I, plasma Mg 0.83 mmol/L; II, plasma Mg 0.84 to 0.91 mmol/L; and III, plasma Mg 0.92 mmol/L. The three groups were identical in terms of age, gender distribution, and degree of obesity. However, both fasting plasma insulin (P < .05) and SSPG (P < .05) concentrations were significantly higher in the tertile (I) with the lowest plasma Mg concentration. Furthermore, there was a significant inverse correlation between plasma Mg and SSPG concentrations (r = -.27, P < .01) in the entire population. These results indicate that variations in the plasma Mg concentration have a relatively modest but significant effect on insulin-mediated glucose disposal in healthy subjects, with lower plasma Mg concentrations associated with increased insulin resistance.