Abstract: An in situ apparatus for measuring a physiological or biological property of a body fluid of an animal includes a body invasive instrument carrying an optical indicator which is exposed to the body fluid and utilizing a spectrophotometric device to generate a color signal or other quantitative measurement representative of the property to be measured which is then transmitted to a receiver remote from the body and converted into a readable output. Real time measurement of rumen pH and other physiological properties in cows and other ruminants utilizes an optical indicator incorporated in a stable porous glass matrix applied to an ingestible bolus. A spectrophotometric device, microprocessor and power source generate a color signal of rumen pH or other property being measured and transmit the color signal to a receiver remote from the ruminant.