The effect of unilateral or bilateral orchidectomy on tissue plasminogen activator activity (PAA), plasminogen activator inhibition (PAI) and plasmin inhibition (PI) was studied in the rat. Orchidectomy was performed in young or adult rats. Unilateral orchidectomy affected only PI in the lungs and brain, 1, 2 and 3 months after orchidectomy was performed in adult rats. Bilateral orchidectomy performed in young or adult rats affected one or more of the parameters studied in one or more of the key organs (brain, lungs, kidneys, heart, aorta), in most cases only 2 and 3 months after orchidectomy. Therefore, the effect of orchidectomy on tissue PAA, PAI or PI was variable and dependent on the extent of the orchidectomy (unilateral or bilateral), the age of the animal at orchidectomy, the time after orchidectomy, and the organ.