A non-invasive method for analyzing the blood-brain barrier includes obtaining a Raman spectrum of a selected portion of the eye and monitoring the Raman spectrum to ascertain a change to the dynamics of the 
blood brain barrier. Also, non-invasive methods for determining the brain or 
blood level of an 
analyte of interest, such as glucose, drugs, 
alcohol, poisons, and the like, comprises: generating an excitation 
laser beam (e.g., at a 
wavelength of 600 to 900 nanometers); focusing the excitation 
laser beam into the anterior chamber of an eye of the subject so that 
aqueous humor, vitreous humor, or one or more 
conjunctiva vessels in the eye is illuminated; detecting (preferably confocally detecting) a Raman spectrum from the illuminated portion of the eye; and then determining the 
blood level or brain level (intracranial or 
cerebral spinal fluid level) of an 
analyte of interest for the subject from the Raman spectrum. In certain embodiments, the detecting step may be followed by the step of subtracting a 
confounding fluorescence spectrum from the Raman spectrum to produce a difference spectrum; and determining the 
blood level and / or brain level of the 
analyte of interest for the subject from that difference spectrum, preferably using linear or nonlinear 
multivariate analysis such as 
partial least squares analysis. Apparatus for carrying out the foregoing methods are also disclosed.