Following acute coronary ligation in intact rats, ventricular fibrillation (VF) occurs in two distinct temporal phases. Phase 1 VF occurs within 30 min, and phase 2 VF occurs during the period when the infarct is evolving, 90-240 min after coronary ligation. The mechanisms of phase 2 VF are not well understood. However, phase 2 VF coincides with neutrophil accumulation in the evolving infarct. We have tested the hypothesis that neutrophil accumulation is necessary for the initiation of phase 2 VF. Male Sprague-Dawley rats (n=10 /group) were randomised to receive 2 mL kg^-1 i.p. of rabbit anti-rat neutrophil antiserum or normal rabbit serum 17 hours before coronary artery ligation. On the following day, rats were anaesthetised with sodium pentobarbitone (60 mg kg^-1). The ECG was recorded using standard limb leads. Rats were ventilated at a rate of 54 strokes min^-1 and 10 ml kg^-1 stroke^-1 with room air. The heart was exposed by intercostal incision followed by rib retraction. A suture (Ethicon Prolene 4/0) was sewn under the left main coronary artery close to its origin, and the ends were threaded through a traction-type occluder that protruded from the chest. Acute single stage coronary artery ligation was performed by tightening the occluder and maintained for 240 min to induce ischaemia and infarction. All rats survived the 240 min occlusion (all VF episodes were successfully terminated by manual cardioversion). Gaussian distributed variables, expressed as mean±SEM, were subjected to analysis of variance followed by Dunnett’s or Tukey’s tests where appropriate. Binomially distributed variables were compared using contingency tables. Antiserum pre-treatment caused neutropenia, reducing circulating neutrophils from 2096±274 x 10³ to 8±8 x 10³ per mL of blood (180 min values; p<0.05). It also blocked neutrophil accumulation in the infarct, with cardiac myeloperoxidase activity falling from 74.7±27.4 to 9±3 mU per mg protein 240 min after coronary ligation (p<0.05). Despite this, there was no difference between control and antiserum-treated rats in the incidence of phase 2 VF (30% in both groups), or ventricular tachycardia (60% vs 80%). There were no confounding differences between control and antiserum-treated rats respectively in postligation blood K⁺ (4.0±0.1 vs 4.4±0.2 mM; p=NS), ischaemic zone size (47±1 vs 46±1% of total ventricular weight; p=NS), or infarct size (81±4 vs 85±2% of the area at risk; p=NS). Likewise, systolic blood pressure (108±7 vs 109±4 mmHg) and heart rate (434±13 vs 439±6 beats min^-1) were similar in the two groups (180 min values shown; p=NS). Phase 1 VF incidences were also similar (30% vs 50%). In conclusion, the lethal ventricular arrhythmias that occur during evolution of the infarct are not mediated by neutrophils.