The single vessel occlusion technique has been applied to venules to investigate the effect of drugs on mass transfer. 2. The filtration coefficient in venules is greater than that in capillaries and in both types of vessel the filtration coefficient is negatively correlated with hydrostatic pressure. 3. Vasoactive drugs have no effect on capillary filtration coefficient. 4. Acetylcholine and isoprenaline do not increase vanular filtration coefficient but histamine, bradykinin, 5-Ht and SPF do and may do so independently of protein leakage.