1. The tripeptide TRH exerts a spectrum of biological activities in both animals and man. Some of these activities have been extensively studied, particularly in psychiatric patients. 2. Behaviorally, TRH appears to increase the sense of well-being, motivation, relaxation, and coping capacity in both normal subjects and patients with psychiatric and neurologic disease. These effects are not disease-specific; attempts to use TRH as a treatment tool have thus been disappointing. 3. Endocrinologically, administration of TRH stimulates the response of TSH; this response has been reported to be blunted in approximately 30% of patients with major depression. However, TSH blunting is not specific for depression, it has also been observed in a variety of other psychiatric conditions. 4. The relevance of these effects for psychiatry in general, and for psychoneuroendocrinology especially, is discussed in this review.