The comparative efficacy of two solutions of bupivacaine, 0.5% plain (isobaric-specific gravity 1.010), and 0.4% bupivacaine (hyperbaric-specific gravity 1.027) in 4% dextrose, for spinal anaesthesia was studied in 67 patients. Both solutions were clinically satisfactory (94% completely successful neural blockade). Hyperbaric bupivacaine blocks a greater number of spinal segments (p less than 0.01), causes a more rapid fall in blood pressure (p less than 0.05), and is more predictable in effect with regard to the number of segments blocked and to the lateralisation of the block. The duration of perioperative analgesia with the hyperbaric preparation is, however, shorter-3.8 hours as opposed to 5.8 hours (p less than 0.01)--but this was likely to be due to the smaller total dosage of bupivacaine used in the hyperbaric group. It is concluded that either solution may be used satisfactorily in clinical practice. The isobaric solution, however, despite its ready availability, is less satisfactory than the hyperbaric because its effect with regard to the extent of blockade is less predictable.