Two prospective studies were performed to assess for how long after the subarachnoid injection of local anesthetics changes in position influence the cephalad spread of sensory blockade. Divergent accounts have been given by other groups. Besides the patient's position other factors may influence the cephalad spread of sensory blockade such as baricity of local anesthetics, speed of injection, dose, volume, barbotage and size of needle. Pashalidou found that after a supine position for 5 or 10 min, followed by Trendelenburg position for 5 or 10 min, there were significant differences in the increase of sensory blockade; the following two prospective studies were carried out with this in mind. METHODS AND MATERIAL. Study 1. Injection of the local anesthetics in sitting position, puncture at L3/4 interspace using a 25-gauge needle, speed of injection 3 ml/10 s, without barbotage. After injection the patients were supine for 30 min, then changing to the Trendelenburg position (n = 20) or the lithotomy position (n = 20), each for 20 min. The spread of blockade was tested by means of pin-pricks in the midline at 5-min intervals. Local anesthetics used were bupivacaine 0.5% with adrenaline (1:200,000) (n = 20) and bupivacaine 0.5% in 8% glucose (n = 20), 3 ml each. Study 2. Intrathecal injection was done as described above, but the speed of injection was 3 ml/6 s. Local anesthetics used were bupivacaine 0.5% with adrenalin (1:200,000) and bupivacaine 0.5% with adrenalin (1:200,000) in 5% glucose, 3 ml each. The patients were kept supine for either 15 or 20 min followed by 20 degrees Trendelenburg position for 10 min. RESULTS. Study 1. The mean spread of sensory blockade with isobaric bupivacaine was 16.95 segments (T6). After the 20 degrees Trendelenburg position the spread of blockade increased by 0.85 segments. After the lithotomy position there was no increase in sensory blockade. With hyperbaric bupivacaine the mean spread of sensory blockade after 30 min in the supine position was 17.3 segments (T5/6). After the Trendelenburg position there was no increase in sensory blockade. After the lithotomy position the sensory blockade spread by 0.4 more segments. This shows that there is no significant increase of cephalad spread of sensory blockade with either isobaric or hyperbaric bupivacaine. Study 2. With isobaric bupivacaine the mean spread of sensory blockade (n = 15) after 15 min in the supine position was 14.4 segments (T8/9). Following the Trendelenburg position the caphalad spread was increased by 0.93 segments (p less than 0.05). With hyperbaric bupivacaine the mean spread of sensory blockade (n = 15) was 16 segments (T7). Following the Trendelenburg position the spread was extended by 2.0 segments (p less than 0.05). After 20 min in the supine position following isobaric bupivacaine the mean spread of the sensory blockade (n = 15) was 15.4 segments (T7/8).(ABSTRACT TRUNCATED AT 400 WORDS)