Adrenaline and noradrenaline increased the perfusion pressure (Pperf) and single glomerulus filtration rate (SGFR) of perfused hagfish glomeruli. Small amounts (0.1 % or 0.5 %) of bovine serum albumin (BSA) in perfusion fluids containing Ficoll 70 did not diminish the loss of colloid from hagfish glomerular capillaries as has been reported for other perfused capillaries. However, replacement of Ficoll 70 with an osmotically equivalent amount (3 %) of BSA appreciably reduced colloid loss. It was concluded that adrenaline and colloids enhanced flow through the urine-forming capillaries. Whereas adrenaline elevated the SGFR, colloid lowered the SGFR probably by a direct effect on the fluid permeability of the capillary walls. The flow-enhancing effect of adrenaline was used to ensure the exposure of urine-forming capillaries to two inhibitors of active fluid transport, ouabain and 2,4-dinitrophenol (DNP). Both substances lowered the single glomerulus filtration fraction (SGFF), probably by affecting a fluid secretion mechanism. In addition, DNP diminished the flow-enhancing effect of adrenaline. This study provides relatively unequivocal evidence that fluid secretion underlies the formation of primary urine by the hagfish.