The significance of TRH for pituitary function is still unresolved mainly due to limitations in determining in vivo hypothalamic TRH release. We therefore examined whether TRH immunoreactivity (TRH-IR) in peripheral blood is an index for hypothalamic TRH release. Peripheral TRH-IR varied between 10 and 55 pmol/l and was similar in euthyroid and hypothyroid rats, but lower in hyperthyroid rats. Destruction of the hypothalamic paraventricular area reduced peripheral TRH-IR, while stimulation of this area increased it. Clearance of TRH during continuous TRH infusion was 1.9±0.2, 3.5±0.3 and 5.9±0.8 ml/min in hypothyroid, euthyroid and hyperthyroid rats, respectively. These and previous data on TRH in hypophysial portal blood indicate that 5–25 pmol TRH/I peripheral blood is of hypothalamic origin. Chromatography revealed that TRH-IR from hypothalamus and portal blood co-eluted with TRH, but in peripheral blood two peaks were found, one of which was authentic TRH. Thus, peripheral TRH-IR alters in experimental conditions and part of it seems to be of hypothalamic origin. However, the presence of TRH-like material in peripheral blood not identical to TRH and the fact that experimental conditions alter TRH clearance indicate that peripheral TRH-IR is not an index for hypothalamic TRH release.