Arthroscopy of the knee was performed using 30 ml single dose intraarticular instillations of 0.5% or 0.25% solutions of bupivacaine (Marcaine). A total of 18 patients (mean age, 34 years), divided into two groups, participated in this study. Venous plasma levels were measured at 0, 10, 20, 30, 45, 60, 90, 120, and 240 minute intervals following a single instillation into the knee joint. All patients had suspected traumatic internal derangement of the knee. Electrocardiogram tracings, blood pressure, and neurologic assessment were monitored at each venous sampling interval or more often if clinically indicated. The type and amount of supplemental anesthesia were also recorded. None of our 18 patients required a general anesthetic because of pain although the following procedures were performed: meniscectomy, plica release, abrasion chondroplasty, loose body retrieval, and limited meniscal repair. A new methodology for the measurement of plasma bupivacaine using the gas chromatograph mass spectrometer is described. Monitoring specific molecular mass fragments allows the measurement of picogram per milliliter levels of bupivacaine. The highest peak plasma concentration occurred 20 minutes after instillation of 30 ml of 0.5% bupivacaine. The 625 +/- 225 ng/ml level was well below the 2,500 to 4,000 ng/ml reported to elicit early subjective CNS symptoms of bupivacaine toxicity. Thus, a single dose intraarticular instillation of 30 ml 0.5% or 0.25% bupivacaine is convenient, efficacious, and pharmacologically safe for routine clinical arthroscopy.