Transvascular escape rate of albumin (TERalb, i.e. the fraction of intravascular mass of albumin (IVMalb) passing to (or during steady state returning from) the extravascular space per unit time) was determined from the initial disappearance rate of i.v. injected radioiodinated serum albumin in anaesthetized pigs during control conditions and during diuretic medication (furosemide i.v. 20 mg/15 min, total 160-200 mg). During diuretic medication TERalb (mean 17.1% IVMalb X h-1, range 11.5-21, n = 6) increased significantly above the control period (mean 12.3% IVMalb X h-1, range 9.5-16.5, P less than 0.05). Pressures in artery, right atrium, hepatic and portal veins did not change significantly from control to diuretic period. TERalb equals the lymphatic return rate of albumin provided the transport mechanisms are filtrative-convective (i.e. no local back transport). Additional measurements in five pigs with proteins of different molecular size confirmed a dominating filtrative-convective transport. The increased TERalb during diuretic medication is best explained by an increased lymph drainage, which may decrease interstitial fluid pressure and thereby increase the transmural capillary pressure difference being essential for a filtrative-convective transvascular albumin transport. Increased lymph drainage may contribute to the therapeutic effect of diuretic treatment in oedema and ascites.