Incubation in vitro of natural (ovine) or synthetic TRF (PCA-His-Pro-NH<sub>2</sub>) with rat blood, plasma, or reconstituted lyophilized serum destroys the biological activity. Inactivation of TRF by normal plasma takes place at 0 °C but at a slower rate than at 37 °C. Plasma obtained from rats several weeks after hypophysectomy and/ or hypophysectomy + thyroidectomy inactivates TRF. TRF activity is not recovered from TRF incubated in plasma when the incubate is treated with NaCl (4 m), ethanol (80% v/v) or HCl (to pH 1). Plasma inactivation of TRF is completely prevented by pretreatment of the plasma with ethanol (80% v/v); heating of plasma at 65 °C for 15 min also destroys the ability of plasma to inactivate TRF. The data reported are consistent with the hypothesis that plasma contains an enzyme (s) which destroys the activity of TRF. Plasma was also found to destroy the biological activity of PCA-His-Pro-OMe and PCA-His-Pro-OH (2 synthetic tripeptides with TRF activity). The biological activity of PCA-His-Pro-OH is destroyed in vitro at a slower rate by plasma than that of PCA-His-Pro-NH<sub>2</sub> or natural TRF. Addition of the dipeptide analogue of TRF, PCA-His-OMe, prevents in vitro the inactivation of TRF by plasma.