The compatibility of granisetron hydrochloride with selected other drugs during simulated Y-site administration was studied. Five milliliters of granisetron 50 micrograms/mL (as the hydrochloride) in 5% dextrose injection was combined with 5 mL of each of 91 secondary additives, including antineoplastics, anti-infectives, and supportive care drugs, in 5% dextrose injection or (if necessary to avoid incompatibility with the diluent) 0.9% sodium chloride injection. Visual examinations were performed with the unaided eye in fluorescent light and in high-intensity monodirectional light to enhance visualization of small particles and low-level turbidity. The turbidity of each solution was measured as well. Particle sizing and counting were performed for selected solutions. Evaluations were performed initially and at one and four hours. Nearly all the test drugs were compatible with granisetron during the four-hour observation period. The granisetron-amphotericin B combination had an unacceptable increase in turbidity upon being mixed. During simulated Y-site administration, granisetron 50 micrograms/mL (as the hydrochloride) in 5% dextrose injection was compatible with 90 of 91 drugs and combination drugs for four hours at room temperature; the exception was amphotericin B.