1 The rate of infusion has been suggested to be important for drug-induced torsades de pointes (TdP) arrhythmias. We investigated the repolarisation-prolonging effects and proarrhythmic properties of NS-7, a neuroprotective drug in development, using two different infusion rates. 2 A fast (5 min intravenously (i.v.)) escalating dosing regimen (0.3 and 3.0 mg kg(-1), n=4) of NS-7 was investigated in anaesthetised control dogs in sinus rhythm (SR). This was compared to a slow infusion (60 min i.v.) of one dose (3.0 mg kg(-1), n=4) NS-7. The similar dosing regimens were investigated in anaesthetised dogs with chronic, complete AV block (CAVB), an animal model of TdP (n=6). 3 No electrophysiological effects were seen after 0.3 mg kg(-1) NS-7. Fast infusion of 3.0 mg kg(-1) caused prolongation of repolarisation, for example, heart rate corrected QT interval (QT(c)): in SR: 6+/-1%; in CAVB: 10+/-7%, which was accompanied by TdP in three of six CAVB dogs. No TdP were seen in SR dogs. 4 Slow infusion did not cause TdP in the same CAVB dogs, although NS-7 caused repolarisation to prolong with a similar magnitude (QT(c): 12+/-7%) as in the fast-infusion experiment. 5 Short-term variability (STV) is a novel parameter for the prediction of drug-induced TdP analysing the beat-to-beat variability of repolarisation. STV was only increased after the fast infusion in CAVB dogs (2.6+/-0.3 versus 6.0+/-1.4 ms, P<0.05), while there was no increase (2.1+/-0.2 versus 2.5+/-1.0 ms) after the slow infusion of NS-7. 6 Peak plasma concentrations attained were lower in slow (0.5+/-0.1 microg ml(-1) after 50 min) than in fast-infusion regimen (2.1+/-0.4 microg ml(-1) after 5 min; P<0.05). 7 The results support the conclusion that limiting peak plasma concentration by decreasing the rate of infusion of NS-7 reduces the proarrhythmic risk despite comparable prolongation in repolarisation parameters. The relevance of STV in predicting drug-induced TdP was confirmed.