The effects of atrial natriuretic peptide on the renal function of the spiny dogfish (Squalus acanthias) in seawater were evaluated. A synthetic mammalian peptide, atriopeptin II (2 micrograms/kg), was injected intravascularly into unanesthetized, unrestrained fish prepared for renal clearance studies. The aortic pressure, glomerular filtration rate (GFR), urine flow rate (V), and urinary excretion of sodium, potassium, and total osmolytes were continuously monitored. Atriopeptin II significantly decreased mean aortic pressure (-12%), GFR (-40%), V (-66%), and the absolute excretion rates of sodium (-47%), potassium (-43%), and total osmolytes (-44%). However, the renal effects of atriopeptin II were temporally dissociated from the vasodepressor effect. Mean aortic pressure decreased quickly and returned to control values approximately 2 h after injection, whereas GFR did not decrease significantly until the third hour after injection. The decreases in renal water and solute excretion in response to atriopeptin could be accounted for by the decrease in GFR, since there were no significant changes in fractional water or solute excretion. Similar decreases in GFR were observed during constant infusions of a lower physiological dose of the peptide (80 pg.kg-1.min-1). The observed antidiuretic and antinatriuretic effect of synthetic atriopeptin in the dogfish contrasts with its putative role as a hormone mediating hypervolemic regulation.