The permeability of toadfish gills and skin to urea and water has been measured in order to investigate the mechanisms behind the pulsatile excretion of urea previously described in this species. A perfused gill preparation was used in the gill studies and isolated pieces of skin mounted in an Ussing chamber in the skin studies. Simultaneously, urea and water permeability was measured in vivo in free swimming fish. In vivo the nonpulsing urea permeability was exceptionally low compared to other teleosts, while the tritiated water permeability was similar to that of other teleosts. The urea permeability increased 30-fold during a pulse while water permeability stayed unaffected. Compared to in vivo, tritiated water permeability was approximately 50% lower in the gills and the skin when measured directly in the isolated preparations. The urea permeability was almost identical between the three preparations. Four out of 20 perfused gill preparation showed a spontaneous urea pulse during perfusion. Several treatments were tested to elicit the pulse artificially but without success. Hormones and drugs tested were: arginine-vasotocin (AVT), 10(-10) M; adrenaline, 10(-7) M; isoprenaline, 10(-5) M; acetylcholine, 10(-7) and 10(-6) M; serotonin, 10(-7) and 10(-6) M; adenosine, 10(-6) M; cortisol, 10(-7) M; and combinations of AVT, adrenaline, and cortisol. Adrenaline and isoprenaline increased tritiated water permeability without affecting urea permeability. Gradually increasing the ammonia levels in the perfusate from 0.1 mM to 1.6 mM caused a slight increase in water permeability but a marked and progressive increase in urea permeability. No indications of an ammonia trapping mechanism in the gills were found. There was no effect of AVT (10(-10) mol l-1) in the urea permeability of the skin preparation while cortisol (10(-7) M) led to a modest increase in urea permeability. Based on a comparison between the in vivo and in vitro preparations used here, we conclude that the urea pulse in a urea-pulsing toadfish occurs through the gills and not the skin. We still do not know which internal mechanism or signal triggers the urea pulse in the toadfish.