The influence of the calcium concentration of the perfusion fluid on the release of striatal dopamine recorded by brain dialysis was investigated. The release of dopamine appeared very sensitive to the calcium concentration of the Ringer. Next we studied whether three different methods known to antagonize the effects of calcium entry, were able to affect the release of dopamine. The conditions investigated were: the use of calcium-free Ringer, infusion of magnesium, and infusion of the calcium-antagonist verapamil. Calcium-antagonism was studied on the day of implantation of the cannula as well as on several days thereafter. It appeared that magnesium infusion was the most effective condition to antagonize the effects of calcium on the release of dopamine. Magnesium infusion was also most effective in preventing drug-evoked voltage-dependent dopamine release (induced by coadministration of haloperidol and GBR 12909). In addition magnesium infusion appeared a potent antagonist of acetylcholine release. In contrast, the dialysate content of aminoacids was not influenced by magnesium infusion.