Provided herein are procedures for rapidly determining oral bioavailability of two or more compounds by administering the two or more compounds to an animal, extracting fluid from an eye region of the animal and determining the amount of each of the two or more compounds in the fluid by mass spectrometry. Such procedures can provide cost advantages for determining oral bioavailability as they allow for (i) utilizing relatively small animals, (ii) extracting multiple samples from a single animal, and (iii) analyzing multiple compounds in a single animal, for example.