Permeability of Silastic tissue expander shells to lidocaine was studied to investigate the feasibility of intraluminal lidocaine injection for pain relief during soft-tissue expansion. Both intact expanders and an apparatus using isolated Silastic membrane segments were used to partition solutions of various lidocaine concentrations, and the rate of diffusion was quantitatively measured using a fluorescence polarization immunoassay. Lidocaine flux was found tofollow Fick’s law of passive diffusion with respect to time, surface area, and concentration gradient for the first 9 hours, with a permeability coefficient of 10.3 ± 2.6 μg (h.cm2.percent)-1 (mean ± SD) and diffusion coefficient of 7.5 × 10-7 cm2/min for an average membrane thickness of 473 ± 23 μm . After 9 hours, the lidocaine flux decreased exponentially, although the concentration gradient across the membrane remained essentially the same order of magnitude. Plasma proteins in the outer bathing solution and methylparaben used as a preservative in the standard lidocaine formulation had no influence on the change in transport flux with time. At the end of the linear portion of the diffusion curve, less than 2 percent of the total intraluminal lidocaine had crossed the membrane. Potential toxicity in the event of implant rupture limits the maximum total lidocaine dose to approximately 500 mg within an expander at any one time. Within these limits, the capacity for lidocaine delivery of 500 mg lidocaine by a 640-cc tissue expander would be only 6 mg during the first 9 hours after administration. While the potential for clinically significant sensory nerve blockade cannot be completely ruled out, we believe that this technique for lidocaine delivery is unlikely to provide significant salutary benefit and advise against it because of the potential for progressive lidocaine accumulation.