The in vivo performance of a 20G copolymer pH sensor, needlelike in configuration, was studied in the normal dog, and dogs made acidotic by the constant infusion of lactic acid, or by the induction of tissue perfusion defects. Sensors were placed at two extravascular sites in the leg, deep subcutaneous (pHe/sc), and intramuscular in the adductor (pHe/im). This pH sensor is a silver wire capped by a H+-specific polymer; it has a built-in reference system. Its electrochemical characteristics and in vivo performance are similar to those of glass pH electrodes. The continuously monitored values were compared with discrete arterial blood gas analyses at 10 to 20 minute intervals. The baseline values in 15 dogs under general anesthesia were: pH/art 7.331 +/- .042, pHe/sc 7.291 +/- .076, and pHe/im 7.265 +/- .102 (mean +/- SD; n = 45 observations each). During metabolic acidosis (lactic acid infusion), the direction and rates of change were similar in pHe/sc and pHe/im. Tissue perfusion defects were induced by moderate-to-severe hemorrhage (single or repeated bleeds) or operative shock (splenectomy and exteriorization of bowel). Both pHe/sc and pHe/im fell sharply, with a more gradual drop in pH/art. In those who survived after infusion of shed blood or dextran-40, pHe recovered rapidly. In the moribund, pHe continued to deteriorate. This pH sensor is a sensitive prognosticator of acid-base changes in the tissue. The in vivo drift is small: 0.008 pH per hour. The placement of the sensor via an intracath cannula in the subcutaneous tissue of the thigh is recommended.(ABSTRACT TRUNCATED AT 250 WORDS)