Unanimity has not yet been reached on the influence of injection speed and needle size on the spread of sensory blockade in spinal anesthesia. While McClure et al. [6] proved that a change in injection speed had no effect on the spread of the blockade, Lanz et al. [4] showed in their investigation that increasing injection speed increases the spread of the blockade. The influence of needle size has hardly been investigated so far. Moore et al. [7] report that the needle size alone has no effect on the spread of the blockade. In this study the influence of injection speed and that of needle size on spread of sensory blockade were considered separately. MATERIALS AND METHODS. Spinal puncture was performed via the midline approach at the L3-4 interspace with the patient in a sitting position. Immediately after the induction of anesthesia the patients were placed in the supine position again. As local anesthetic 15 mg (3 ml) bupivacaine 0.5% with epinephrine 1:200000 was administered. The aim of the study was to find out how far injection speed and size of the spinal needle influenced the sensory spread in isobaric spinal anesthesia. The height of sensory blockade was assessed by means of the pin-prick method in the midline, and the onset of analgesia was determined as height of spread. The injection speeds for the local anesthetic solution were 0.25, 0.5 and 1 ml per second. The needle sizes were 22, 25, and 29 gauge. Each group consisted of 15 patients. RESULTS. It turned out that slow (0.25 ml/s) and the fast (1 ml s) injection was associated with a significantly higher level of analgesia than the medium one (0.5 ml/s). Cephalad spread was to T7 in the first two groups, while the medium injection speed only achieved a level of T9. The results are statistically significant (P less than 0.05). The comparison of different needle sizes (22, 25, and 29 gauge) used for spinal anesthesia showed a higher spread of the sensory blockade with increased diameter of the spinal needle given a constant injection speed. With the 22-gauge needles (n = 15) sensory blockade extended on average to T7, with 25-gauge needles (n = 15) to T9, and with 29-gauge needles (n = 15) to T10. It has to be mentioned, however, that with the 29-gauge needle the standard injection speed of 0.5 ml/s could not be achieved because of the small inner diameter. The differences between 22- and 25-gauge needles are statistically significant (P less than 0.05). DISCUSSION. No direct relation could be proved between the different injection speeds and the spread of the blockade. Our results are hardly comparable with those of other investigators, since other groups have used different local anesthetics or performed investigations in vitro. The use of large spinal needles is associated with spread of the spinal block to a significantly higher level than is achieved with thinner needles.(ABSTRACT TRUNCATED AT 400 WORDS)