The first acidizing of Zechstein Dolomite in the Polish Lowlands was carried out in 1961. In the initial stage, a tendency existed toward multistage acidizing, and preheated acidizing fluid was used. In a preliminary operation, an acid having the concentration 12-36% was used for washing out the borehole. No additives were utilized except formalin for corrosion inhibition. In order to increase the absorption capacity, hydraulic perforations were used. The composition of acidizing fluid was determined in a preliminary study for any individual reservoir. From laboratory study, it was concluded also that the required time of 3-4 hr was sufficient for the exhaustion of acid. The pre-heating of the acid at the bottom of the hole by exothermic reaction with metallic magnesium was abandoned in the later stage of operations.