Twenty hyperprolactinemic patients who entered a randomized parallel-group, double-blind, double-dummy study were investigated regarding safety, tolerability and efficacy. Half of the patients received 5 mg Parlodel(R) SRO plus placebo for Parlodel while the other half received 2.5 mg Parlodel b.i.d. and placebo for Parlodel SRO for a period of 15 days. In the second following period of 15 days, the daily dose was increased to 10 mg Parlodel administered either as a single dose of Parlodel SRO or two doses of Parlodel 5 mg. The plasma prolactin levels, clinical signs and symptoms of hyperprolactinemia, physical examination, blood pressure, heart rate assessments and adverse events were recorded during the study. Complete blood count, blood chemistry and standard ECG were performed before and at the end of treatment. In conclusion, both formulations are equally efficacious, well tolerated and safe. Due to the comfort of once-a-day administration and the excellent compliance, one could recommend to replace the b.i.d. or t.i.d. administration of Parlodel with the once-a-day Parlodel SRO in hyperprolactinemic patients.