The better to comprehend isobaric spinal anaesthesia, we constructed a thermoregulated model of the subarachnoid space. With it we investigated the six not exactly isobaric local anaesthetics bupivacaine, carticaine, lidocaine, mepivacaine, prilocaine and tetracaine, in order to determine the effect of the following factors on their spread: injected volume, position, density, temperature of the injected solution, rate of injection, barbotage, direction of the spinal needle and its opening. The injected volume was most important. The small differences in density were of consequence in the vertically mounted model. Higher rates of injection increased the spread. The remaining factors were not relevant. The investigation corroborates the clinical observation that spinal anaesthesia with approximately isobaric solutions is a well controllable technique.