An in vivo determination of the presence or concentration of an endogenous or exogenous substance by photoacoustically assaying the substance in the eye and correlating the presence or concentration of the substance in the eye to the presence or concentration of the substance in the blood, without removing a tissue or fluid sample from the body for assay. The eye, unlike other body sites such as the skin, has a relatively constant pressure and temperature, providing an additional utility for the inventive method.