with the invention o f the ion-selective electrode, ionic magnesium (img), the biologically active form of mg, is a common blood assay. there is some evidence that img has considerable within subject variability. to further assess this, blood samples were collected from 13 healthy volunteers 6x/day (7:00 - 22:00, every 3 hours) for 3 consecutive days and analysed for img (nova stat 8 analyser). individual ranges averaged .08 mmol/l (range .05 to .14). coefficients of variation (cv) ranged from 3% to 7% (mean 4%) while analytical variation was determined to be 2.3%. biological variability thus accounts for almost half of the variability which is clinically significant as 9 of the 13 subjects recorded at least one value below a reference range of .46 - .60 mmol/l. a significant within day variation (p<.001) was noted, with differences between 7:00 and 10:00 as well as 10:00 and 22:00. between day variations were not significant (p=.56). a plausible explanation of this data is that img has a circadian rhythm, but a sinusoidal curve of best fit computed for each subject did not correlate highly to img (r=.20). thus, cautious interpretation of img values is warranted until future research determines the nature of img variability.