BACKGROUND: Multilumen catheters are commonly used in critically ill children. Their use, however, is associated with significant morbidity. We studied the simultaneous administration of incompatible drugs using a new triple-lumen catheter with decreased length and port to port distances. METHODS: Ten domestic swine, 10-20 kg in weight, were divided into two groups of five. Total parenteral nutrition was administered through the distal port and phenytoin was administered as a bolus and as an infusion in each group. Samples were taken from two sites during the bolus and at 1, 5, and 15 min during phenytoin infusion. Histograms were generated for particle size and concentration. Samples were also examined under the microscope for particles. RESULTS: Histograms of particle size did not show any alteration of the histogram that would suggest particle size > 2 µm in diameter in the study or control samples. No particles were identified by phase microscope, light microscope, or Wright stain smear. CONCLUSIONS: The use of a triple-lumen catheter with a distance of 0.4cm between the proximal port and the medial port and 1.3 cm between the medial port and the distal port, for the in vivo simultaneous administration of incompatible solutions does not result in precipitates large enough to cause adverse clinical effects.