The accuracy of drug delivery by drop-counting infusion pumps depends among other considerations, on the assumption that drop volume is constant. This assumption may hold true in most cases, except for surface active agents. Lowering the surface tension causes drop size reduction and a decrease in actual drug flow when delivered by drop-counting infusion pumps. To determine the number of drops per milliliter of intravenous solutions, a system comprising an infusion set connected to the bottom of a vertically mounted 5-ml graduated pipette has been developed. A device containing a light source and a photoelectric cell was interfaced to an ATARI 104ST microcomputer by means of an analog-to-digital converter and then attached to the filter of the infusion set. An assembler routine detected the falling drops and a main BASIC program calculated the cumulative number of drops and the instantaneous drop rate. Preliminary experience showed that this surface tension lowering effect may account for decreases of up to about 25% in drug delivery when drop-counting infusion pumps are used and thus may have practical clinical consequences.