The basal levels of prolactin (PRL) and their changes after i.v. thyrotropin releasing hormone (TRH) administration after bromocriptine (BCT) pretreatment (BCT/TRH test) were monitored in 12 patients with young-onset Parkinson's disease (PD) (before 40 years of age) and 10 patients with older-onset PD (after 40 years of age), as well as in two groups of healthy subjects (10 persons in each), age-matched with older-onset (control A) and young-onset (control B) parkinsonians. The basal PRL levels were normal in both groups of patients. When given after BCT, TRH induced a significantly lower PRL increase in older-onset parkinsonians than in controls. This response was even more blunted in young-onset patients, being significantly more attenuated than in older-onset PD patients.