Summary: Ten healthy male volunteers received two doses of tobramycin (2 mg/kg) in a crossover fashion, first by intravenous piggyback (IVPB), then by the CRIS infusion system after a washout period. Serum samples were drawn both during and after the infusions. Twenty-four-hour urine collections were assayed for tobramycin. Residual fluid from the lines of both delivery systems was measured and assayed for tobramycin concentration. All samples were run in duplicate, using an enzyme-multiplied immunoassay technique assay. The results indicate that there was a statistically higher amount of drug delivered via the CRIS system (98.3 \pm 0.3% versus 90.4 \pm 2.3%). No significant difference was found in urinary recovery between the two groups. Peak serum levels were significantly higher with the CRIS system, with 8/10 subjects having at least one serum level >10 mUg/ml, as compared to 0/10 when given by IVPB. Peak serum levels occurred at 30 min in all subjects given tobramycin through the CRIS system, compared to 50–60 min when delivered by IVPB. This difference in peak serum levels is primarily related to the rate of drug delivery and to the difference in the dose delivered to each subject. The significance of the serum concentration profiles is discussed.