1 L(-)- and D(+)-bupivacaine in eight different concentrations from 0.06 to 7.69 mmol/l (0.002--0.25%) and physiological saline were given intradermally to seventeen volunteers, using a double-blind technique. 2 Local colour changes were observed as 'nil', 'pink' or 'pale' and analgesia to pinprick was assessed every 10 min to give an estimate of apparent in vivo potency and duration of action (50% recovery). 3 Both isomers were vasodilator in all cases at 7.69 mmol/l, but the incidence of vasodilatation waned with decreasing concentration, more rapidly with L(-)-than with D(+)-bupivacaine. 4 Only L(-)-bupivacaine showed a vasoconstrictor effect. This was maximal at a concentration of 0.48 mmol/l, when the incidence of pallor was 92%. 5 L(-)-bupivacaine had a longer duration of analgesic action than the (D+)-isomer from 0.48-3.84 mmol/l; this was reflected in a higher apparent in vivo potency.