We measured the serum concentration of lidocaine after transversus abdominis plane (TAP) block with 40 ml of 1% lidocaine in 12 patients under general anesthesia, using a fluorescence polarization immunoassay. The peak mean serum concentration of lidocaine occurred 30 min after the block (3.6 +/- 0.7 microg x ml(-1)). The highest concentration of lidocaine (5.microg x ml(-1)) was recorded 15 min after the block. These results indicate that a TAP block can potentially cause systemic toxicity of a local anesthetic. The analgesic effect of the TAP block may partially depend on the rise in serum concentration of the local anesthetic.