New analgesics administered into the spinal space are currently being investigated. Unfortunately, the movement of the drugs in the spinal fluid (fluid in the spinal space), is not understood. Therefore, the investigation of new drugs require a "hit or miss" experimental dosing regimen to establish clinically useful doses. The purpose of this protocol is to measure the changes in the concentrations of two drugs, morphine and fentanyl, administered through one spinal needle with samples of spinal fluid removed through a second spinal needle. The measurements of spinal fluid drug concentrations will be used to support a study to develope a computer program, which will predict the movement of drugs in the spinal fluid. Predicting the movement of drugs in the spinal fluid is important because it can speed up the understanding of how to use new spinal drugs.