The objective of this study was to report the data on predictability of complete delivery of three parenteral antibiotics (ceftriaxone, chloramphenicol, and tobramycin) after retrograde infusions. The antibiotic doses were infused in vitro at 60 ml/h using a macrobore retrograde tubing. Serial samples were collected over a 2-hour period. Drugs were measured by specific and accurate high-performance liquid chromatographic or enzyme-mediated immunoassay techniques. For the delivery of greater than 95% doses, the predicted time was 30 minutes but the actual times ranged from, 30–45 minutes for various drugs. This delay of up to 50% for certain drugs should be recognized in recommending appropriate times of blood collection to determine peak serum concentrations of antibiotics. Retrograde infusion method may be useful in selected patients but its merits and limitations relative to usual antegrade infusion techniques should be fully understood.