All the venomous snakes featured were authentic and the cast actually did have to interact with them for filming. Only in the shot where Strother Martin grabs the king cobra's head during the show was a puppet snake used.
The cast was considerably nervous about interacting with the live snakes. Star Heather Menzies said she was able to "steel" her nerves to work with her scaly co-stars during filming and forgot her original apprehension. However, after the production wrapped she said she reverted back to her original fear of snakes.
The mongoose in the movie was actually a tayra, a member of the weasel family native to tropical areas of Central America and South America.
For the scene where Dr. Stoner first injects David, Strother Martin actually makes a real hypodermic puncture on Dirk Benedict's arm with the needle.
Amongst the 100 snakes used were rattlesnakes, boa constrictors, pythons, king cobras, blue racers, hognose snakes, and two-headed Florida pine snakes.