Selected teleostean (bony) fish species of the family Batrachoididae (toadfishes and midshipmen) possess high titers of all enzymes of the ornithine-urea cycle in their livers. These species have proven valuable in understanding the short-term regulation of urea synthesis, urea permeability, and transport across epithelial tissues, and how urea synthesis and excretion have evolved among vertebrates. One species in particular, the gulf toadfish (Opsanus beta), has been shown to rapidly switch from ammonia excretion to urea synthesis and excretion during a variety of stress conditions (including confinement). The transition is accompanied by an upregulation of hepatic glutamine synthetase activity, and a switch to pulsatile urea excretion from the anterior end of the fish. In fact, a single day's excretion can be voided in a period of < 3 h. Hypotheses on the environmental significance of these patterns of urea synthesis and excretion are discussed.