2-Thiazoline - 2 - ammonium-4-n-butyl-1,2-diphenyl-3,5-pyrazolidinedionate (thiazolinobutazone, LAS 11 871, Fordonal) has been compared with phenylbutazone in a variety of pharmacological tests with a view to side effects and toxicity. Thiazolinobutazone was the less toxic (acute LD50) and the less ulcerogenic molecule of the two and, in contrast to phenylbutazone, was not antidiuretic. Some signs of activity on the autonomic and somatic nervous systems were seen following high doses of thiazolinobutazone which appeared to be related to inhibition of cholinesterases.