The basal prolactin (PRL) levels on five different days, the PRL responses to thyrotropin-releasing hormone and to domperidone, and the thyroid-stimulating hormone (TSH) responses to domperidone were studied in 75 regularly menstruating women and 44 patients with moderate hyperprolactinemia. It was found that, for the entire sample, the responses to each of the stimuli could be described by a continuous function of the basal PRL levels. The present work provides evidence for the following conclusions: 1) The PRL responses to thyrotropin-releasing hormone and to domperidone merely bring additional diagnostic information relative to basal PRL levels in the occasional patients with macroprolactinemia; 2) there is a continuous spectrum of lactotroph activities in women, ranging from normal secretors through an intermediate group of hypersecretors (with progressively increased serum PRL levels and decreased responsiveness to stimuli) to full-blown prolactinomas; and 3) idiopathic hyperprolactinemia is a heterogeneous entity that includes the above intermediate group of patients, women with macroprolactinemia, and patients with undiagnosed prolactinomas.