Aestivation in African and South American lungfish (Protopterus and Lepidosiren, respectively) is associated with elevations of extracellular osmolarity. Osmotic shrinkage of Protopterus red blood cells (RBCs) caused a small but significant stimulation of the Na influx that was amiloride-sensitive. suggesting involvement of the Na+/H+ exchanger (NHE). The associated in vitro regulatory volume increase was insignificant within a time frame of 120 min, but the shrinkage-activated Na+ influx may be sufficient for slow regulatory volume increase during aestivation in vivo. Osmotic swelling of the RBCs induced an incomplete regulatory volume decrease that was statistically significant after 180 min. The RBCs of Protopterus were very large (mean cellular volume of 6939 +/- 294 microm3) and possessed 23,066 +/- 7,326 beta-adrenoceptors cell(-1) with a Kd value of 6.1 +/- 3.2 nM. The number of receptors per unit surface area of lungfish RBCs was calculated to be twice that of trout RBCs and 70% that of cod RBCs. There was, however, no adrenergic stimulation of the NHE in either Protopterus or Lepidosiren. Acidification of the extracellular medium also failed to activate the NHE.