Abstract The effect of haloperidol treatment on human plasma levels of magnesium (Mg), calcium (Ca) and phosphorus (P) was assessed. Haloperidol treatment significantly reduced plasma Mg but not plasma Ca or P levels. This finding contrasts with studies using other neuroleptics where reductions in both Mg and Ca concentrations were observed. The addition of lithium to haloperidol treatment resulted in an elevation of plasma Mg, but not Ca or P, to levels significantly greater than those at baseline. The reduction of Mg levels by haloperidol may reflect the ion's involvement in the pathology of psychosis and/or in the mechanism of generating extrapyramidal side effects.