Activin A and follistatin are normally present in relatively low amounts in the circulation. Heparin administration elicits a rapid and robust release of these proteins, although this phenomenon is poorly defined. In the present studies, the response to heparin administration was evaluated in the plasma of adult ewes in terms of whether it was dose-dependent, could be neutralized, was responsive to multiple stimulation, and the nature of the activin A and follistatin released. Activin A and follistatin were rapidly released by heparin in a dose-dependent manner (25, 100 or 250 IU/kg), with differences in the response as adjudged by peak concentration, timing of the peak and area under the curve. The heparin response could be blocked by pretreatment with protamine; conversely protamine injection alone (2 mg/kg) elicited release of follistatin but not activin A. Repeat administration of heparin at three-hourly intervals resulted in activin and follistatin responses to each injection, but each subsequent stimulation increased and extended the responses, consistent with saturation of the heparin clearance mechanism. Size exclusion chromatography of plasma samples confirmed that the majority of activin and follistatin released by heparin was a complex, whereas follistatin released by protamine was unbound. These data are consistent with a large pool of activin A and follistatin resident on extracellular matrices, with the rapid response implicating the vascular endothelium as the prime site of release following administration of these commonly used anticoagulant therapies.