The authors were interested to find whether the course of sensory and motor blockade in isobaric spinal anesthesia was determined by the dose or the volume of the anesthetic agent. In a randomized double-blind study in 60 patients, each underwent isobaric spinal anesthesia with 17.5 mg bupivacaine. In three groups of 20 patients, this dose was administered as 3.5 mg bupivacaine 0.5%, 7 ml bupivacaine 0.25% or 10 ml bupivacaine 0.175%. The development, spread and regression of sensory block (anesthesia, analgesia) and motor block (Bromage scale, rectus abdominis muscle test) were determined. The clinical data recorded in the three groups were comparable. The rate of development, the maximal spread or intensity, and the regression of sensory and motor blockade did not differ in the three groups. The only difference was that the complete regression was shorter following 10 ml bupivacaine 0.175% (P less than 0.05). It is therefore concluded that the dose, and not the volume, determines the course of sensory and motor blockade of isobaric spinal anesthesia.