I was told about this fantastic paper by a fellow grad student many years ago. I thought I'd lost the reference, but I found it again:

Lysergic acid diethylamide: its effects on a male Asiatic elephant
by West, L. J., Pierce, C. M. & Thomas, W. D.
Science (1962) 138:1100-1104.

In this article, they found that a single, intramuscular dose of 297 mg of LSD caused sudden death in an elephant tested in Lincoln Park Zoo, Oklahoma City.