A new series of imidazolyn-5-one derivatives were synthesized. These compounds were screened for their analgesic, anti-inflammatory and antipyretic activities, as well as for their ulcerogenic potential, for behavioural effects and acute toxicity. Some of them showed higher analgesic activity than phenylbutazone (PBZ), but lower anti-inflammatory activity. Their ulcerogenic effect was lesser than that of the reference drug.