The basal and TRH-induced prolactin (PRL) and TSH secretions were examined in 14 children aged 4 to 13 years with newly diagnosed insulin dependent diabetes (IDD) within three weeks after diagnosis. Basal PRL levels did not differ from the values of control. In response to TRH, 6 out of 14 patients showed an exaggerated PRL response and 8 a normal PRL response. The basal and TRH-induced TSH secretions were normal, while plasma triiodothyronine (T3) and thyroxine (T4) concentrations were significantly (p less than 0.001, p less than 0.02) lower in patients than in controls. These findings suggest that a significant proportion of children with newly diagnosed IDD has an exaggerated PRL response to TRH, and TSH secretion remains unchanged despite significant decreases of circulating thyroid hormone levels.