beta-Adrenergic agonists are useful for the emergency treatment of asthma. Recently, magnesium sulfate (MgSO4) has also been shown to be efficacious in this situation. beta-Agonists have unwanted cardiovascular and metabolic actions: increased systolic blood pressure, corrected QT interval (QTc), serum glucose and insulin, and decreased RR interval, diastolic blood pressure, serum potassium, phosphate, and calcium. As beta-agonists and MgSO4 quite possibly will be used in combination, we sought to determine how MgSO4 would affect these actions. Healthy young male adults were administered two doses of terbutaline sulfate, 0.25 mg subcutaneously, 30 min apart on two separate occasions, in a randomized, double-blind fashion. On one occasion, 4 g of MgSO4 was administered intravenously over the same 30-min period. On the other, normal saline solution was given as a placebo. Cardiovascular and metabolic variables were measured sequentially for 2 h. Data at 60 min with p values given for a summation of all time points are as follows: MgSO4 increased terbutaline's effects on the RR interval by 0.09 s, p < 0.0001; QTc interval by 0.01 s, p < 0.0007; diastolic blood pressure by 8 mm Hg, p = 0.0001; serum calcium by 0.13 mg/dl, p = 0.01; and glucose by 9 mg/dl, p < 0.0001. MgSO4 also mitigated the systolic blood pressure elevating the effect of terbutaline by 5 mm Hg (p = 0.007). The magnitude of the response potentiations was modest. We conclude that combining terbutaline and MgSO4 is unlikely to result in serious short-term adverse events, if used acutely in patients with relatively normal cardiac and metabolic function. MgSO4 may act by potentiating the effect of beta-agonists on magnesium requiring enzymes such as adenyl cyclase.