In a study of myocardial magnesium and calcium levels in 1236 samples of heart muscle taken at necropsy in 28 centres in England and Wales, samples from patients who died from ischaemic heart disease had a mean Mg level 23 microgram/g lower and a mean Ca level 4 microgram/g higher than the mean for the total series after allowance had been made for a number of relevant factors. Mg levels were slightly higher and Ca levels slightly lower in sudden than in lingering deaths. There was no evidence of any association between the tissue levels and the levels of Mg or Ca in domestic tap-water, but there was a weak association between mean tissue Mg level and IHD mortality rates in the various towns.