Selective cyclooxygenase (COX)-2 inhibitors are a new class of nonsteroidal anti-inflammatory drugs (NSAIDs) which may provide safer and more potent analgesia than currently available non-selective agents. Development of COX-2 inhibitors for veterinary use would be facilitated by assays to evaluate their biochemical effectiveness in the dog. Canine COX-1 and COX-2 isoenzymes were found to be differentially sensitive to in vitro inhibition by NSAIDs using a canine monocyte/macrophage cell line which constitutively expresses COX-1, but can be induced to express COX-2 when incubated with lipopolysaccharide. Inhibition of prostaglandin E\sb2 (PGE\sb2) synthesis by each NSAID was measured by enzyme immunoassay and attributed to specific COX-1 or COX-2 activity through assessment of COX mRNA expression using Northern analysis and RT-PCR. Meloxicam and tolfenamic acid demonstrated preferential inhibition of COX-2; with meloxicam inhibiting COX-2 activity 12 times more effectively than COX-1 activity. Effects of carprofen and ketoprofen approached equipotency against both isoenzymes.