In any 24-h period the body temperature (Tb) of the central Australian agamid lizard, Ctenophorus nuchalis, may vary from 13 to 45C; the mean preferred Tb is 37C. We have analyzed surfactant-type lipids in lizards that underwent rapid changes in Tb from 37C to 14, 19, 27, or 44C. Lipids were extracted from lung lavage and lamellar body fractions, and phospholipids and cholesterol components were measured. There was no change in either the total amount or relative proportions of the different classes of phospholipids, but cooling increased the cholesterol content of lavage. An increase in the cholesterol: phospholipid ratio was evident within 2 h of cooling to 19C and was maintained for at least 48 h. The ratio increased from 8% at 37C, to 15% after 4 h at 19C, and 18% after 4 h at 14C. Possibly the increase in cholesterol promotes fluidity and absorption of surfactant within the alveoli of lizards with low Tb. Cold lizards collapse their lungs during prolonged periods of apnea and the surfactant may prevent the epithelial walls from adhering.