The effects of protirelin (thyrotropin-releasing hormone [TRH]) administration on the release of five pituitary hormones (thyrotropin [TSH], prolactin [Prol], growth hormone, luteinizing hormone, and follicle-stimulating hormone [FSH]) were examined in 45 patients with major depressive disorder and 32 healthy volunteers. Although mean pituitary responses to protirelin in depressed patients and controls appeared to be comparable, depressed patients had higher SDs in all cases. Twelve patients (26.7%) but no controls had two or more abnormal hormonal responses to protirelin administration. The use of several nonparametric analyses revealed significant differences in patterns of hormonal response between depressed patients and controls for TSH, Prol, and FSH. These findings support the hypothesis that increased variability of neuroendocrine responsiveness represents a fundamental aspect of physiologic function in patients with endogenous depression.