Magnesium deficiency has been implicated as a possible causative factor in premenstrual tension (PMT). We have assessed serum and red cell magnesium concentration in nine normal premenopausal women and 26 PMT patients, using atomic absorption spectrometry. The following means +/- SEM were obtained from serum and red cell magnesium, respectively, in mg/100 ml: normal subjects: 1.7 +/- 0.04 and 4.5 +/- 0.25 PMT patients: 1.8 +/- 0.05 and 3.1 +/- 0.24. Mean red cell magnesium level was significantly (p less than 0.01) lower in PMT patients. Red cell magnesium determinations should be included in the evaluation of PMT.