Abstract: It has been surprisingly found that very accurate measurements of mass transfer can be made rapidly by permitting diffusion of an agent desired to be measured into a small, known volume of receiver or out of a known volume of donor, then rapidly pumping or flushing (“pulsing”) the receiver with a known volume of fluid. More specifically, a novel method of transferring small quantities of a contained material (either dissolved or suspended) between two media, based on such pulsing, hereinafter called pulsatile microdialysis (PMD), is disclosed. In a preferred embodiment, one medium (the dialysate) is inside a small, permeable tube (microdialysis probe window) and the other (external medium) is outside. The transfer of material between the two media can be utilized, for example, to sample drug concentrations in the external medium, or the release of drugs from systems within the dialysate, or for other measurements as disclosed herein.