Lorazepam is a long-acting benzodiazepine that interacts with a high degree of affinity for the GABA receptor complex. This high affinity binding in turn leads to a prolonged duration of action. There are no published clinical studies documenting its duration of action in dogs or its ability to control seizures. The purpose of this study was to compare the duration of seizure control of lorazepam with diazepam in 16 dogs presenting in status epilepticus or with active cluster seizures. Previous seizure history and anticonvulsant therapy was not a consideration for inclusion into this study. Animals were excluded if there was a known metabolic, toxic or traumatic cause of the seizure. Dogs were randomly assigned to receive either lorazepam (0.2 mg/kg IV) or diazepam (0.5 mg/kg IV), and the clinicians were blinded as to which drug they were administering. The duration of the study was 12 hours from the time of drug administration, and the animals were monitored for any indication of seizure activity, including generalized motor activity, focal motor activity (e.g., movement of facial or limb musculature) and change in the level of consciousness. The study ended at 12 hours post-study drug administration or when the dog seized before the end of the 12 hour study period. The results indicated no significant difference between lorazepam versus diazepam with regard to median seizure-free interval (2.8 h for diazepam versus 3.4 h for lorazepam, p=0.58 by log rank test), or with regard to percent seizure-free for the duration of the observation period (1/8 for lorazepam versus 3/8 for diazepam, p=0.51 by Fisher's exact test). There was also no difference between the 2 drugs regarding the number of animals in which seizures were initially controlled (6/8 in each group). Lorazepam used at this dose does not appear to result in a significant increase in duration of seizure control for dogs with status epilepticus and cluster seizures. Additional studies may be warranted using higher doses of lorazepam.