Comparative studies were performed in vitro to test the advocated superiority of infusion over intermittent injection of aminoglycosides. Pseudomonas aeruginosa was exposed to constant and to continuously decreasing (simulating in vivo kinetics) concentrations of gentamicin. In comparing the effect with similar area-under-the-concentration-vs.-time curves, a substantial difference in killing and regrowth could not be demonstrated. Regrowth occurred only when the gentamicin concentration had continuously decreased below one fourth of the minimal inhibitory concentration for greater than 2 hr. Exposure of P. aeruginosa to gentamicin for 30 min was followed by persistent suppression of bacterial regrowth for 1.4-1.9 hr. Thus, intermittent exposure of P. aeruginosa to gentamicin is as effective as constant exposure in vitro. The demonstrated persistent postantibiotic effect might cover in part the periods between intermittent doses of gentamicin in vivo as well as in vitro.