The effect of a one-segment (L-2) laminectomy on spinal cord blood flow (SCBF) was determined by the reference sample method using isotope-labeled microspheres. The SCBF was measured before laminectomy (control) and at 15 minutes postlaminectomy with the dura exposed (Series 1), 1 hour postlaminectomy with the laminectomy site closed (Series 2), 24 hours postlaminectomy with the laminectomy site closed (Series 3), and 24 hours postlaminectomy with the dura exposed (Series 4). With the laminectomy site open, SCBF was significantly depressed (22% to 45%) along the entire length of the spinal cord at 15 minutes postlaminectomy. At 1 hour postlaminectomy (with the laminectomy site closed), SCBF approached control values, although areas with significantly lowered flow were still observed in all portions of the spinal cord. By 24 hours postlaminectomy, SCBF had returned to prelaminectomy levels. However, if within 1 hour preceding the 24-hour SCBF measurement, the laminectomy site was reopened, SCBF tended to fall at and caudad to the laminectomy site. These data indicate that laminectomy can cause a significant decline in SCBF. At the present time, the mechanism(s) for this laminectomy-induced depression of SCBF are unknown, although a temperature-induced vasoconstriction is suspected.