A miniaturized Fiberoptic Pco2probe permits direct measurement of tissue Pco2on nonkeratinized tissue surfaces without the heating effects produced by transcutaneous Pco2sensors. This study of anesthetized dogs compared Pco2measured at the palpebral conjunctiva (Pcjco2) with Paco2and mixed venous (Pvco2) measurements during normovolemic normotension and hypovolemic hypotension. During the control period, the average Pcjco2was 3 ± 1 (SEM) torr greater than Paco2(r = 0.98) when the latter ranged from 20 to 80 torr. There was a close association (r = 0.94) between Pcjco2and Paco2when the cardiac index (CI) was greater than 2.0 L/minm2. However, as CI decreased below this value, PCjCo2and Paco2were less well correlated (r = 0.69). Pcjco2was closely associated with Pvco2at all stages of the experiment, both above (r = 0.95) and below (r = 0.82) a CI of 2 L/min m2. Fiberoptic conjunctival Pco2monitoring seems promising as a noninvasive measure of physiologic status.