Ethanol replacement by CO2 of glutaraldehyde-fixed and ethanol-dehydrated rabbit articular cartilage specimens was monitored with both gas chromatograph and alcometer prior to critical point drying (CPD). The surface structure of the patellar specimens was also systematically registered with a semiquantitative scanning electron microscopic method. After a 2 h interval, when about 28 microliters of ethanol/15 min CO2 extract was removed, the articular surface was smooth, although small areas of striated surface and superficial splits were present. A long-term CO2 treatment (16 h) removed ethanol completely, but increased superficial splitting of the articular surface after CPD. Air-drying of the specimens gave rise to inferior preservation of the cartilage: large areas with pitted and leafy surface qualities, but no superficial splits, were present on the surface. It was evident that prolonged ethanol replacement by CO2, prior to CPD, degraded surface structure of the articular cartilage which should be taken into consideration in the planning and design of experiments. Ethanol removal by CO2 could conveniently be monitored by an alcometer.