The authors evaluated transcutaneous Pco2(PtcCo2) and Po2(PtcO2) electrodes in 25 infants. Their diagnosis were severe hyaline membrane disease (HMD) (18), aspiration syndrome (3), severe hydrops, (3) persistent fetal circulation (6), and the others, congenital pneumonia, congenital plural effusion, pulmonary hemorrhage. In almost all, the cardiovascular system was compromised, i.e., PDA with congestive heart failure and shock. PtcO2electrode was heated to 43.5°C while PtcCO2electrode was not heated. Simultaneous arterial blood pressure (ABP), pH, arterial blood gases were obtained with the transcutaneous gas measurements. The data were analyzed first dividing all the paired arterial and transcutaneous gas tensions into those with and without cardiovascular drugs (dopamine, isoproterenol), and second, the paired values were divided into those taken (a) during severe acidosis (pH < 7.25), (b) hypotension (<2 SD) of normal, and (c) hypotension and acidosis. These data show: (1) the unheated PtcCO2and heated PtcO2accurately correlated with the simultaneous arterial measurements; (2) Ptcco2reflects tissue Pco2; (3) drugs affect both Ptco2and Ptco2; (4) elevated Ptcco2dissociating from the simultaneous Paco2in neonates with cardiovascular compromise results from decreased tissue perfusion. These data suggest that transcutaneous gas sensors perform dual functions; first, as gas monitors in patients without cardiovascular alterations, and second, in patients with cardiovascular compromise, Ptcco2reflected tissue perfusion and Ptco2monitored oxygen delivery to the tissues.