The Black Pit Of Dr. M (1959)

Starring Gaston Santos, Rafael Bertrand, Mapita Cortes, Carlos Ancira, Carolina Barret
Directed by Fernando Mendez
(actor & director credits courtesy IMDB.com)

A doctor at a sanitarium obtains a promise from a dying friend to show him the way to enter the land of the dead and return, but he has no idea how this will occur.

Another fine Mexican horror film from Mendez, this effort has some fine photography and a memorable music score, and a rather unique premise and story.  The setting of the spooky sanitarium also allows for some menacing developments, with a homicidal patient who flies into rages when not comforted by the soothing melody of a musical box.  Cortes, introduced in the story as the estranged daughter of the dying man who promised Bertrand the secrets of the afterlife, is very beautiful and provides the center of a love triangle when Bertrand becomes obsessed with her, and Santos also pursues her, both having mutually dreamed of the other.  Also, key to the film is Ancira's orderly at the sanitarium, whose actions also weave into the themes of doomed fates in the narrative.  This is a first-class production all the way, with Mendez committed to delivering a grim tale of the supernatural, with no hint of comic relief.

Isle Of The Snake People (1971)

Starring Boris Karloff, Julissa, Carlos East, Rafael Bertrand, Yolanda Montes
Directed by Juan Ibanez
(actor & director credits courtesy IMDB.com)

A captain of police tries to establish law and order on an island where the people practice voodoo rites, but in return his men are assaulted and killed, and a powerful curse is placed upon him.

Here's another of the Mexican horror films actor Boris Karloff filmed scenes for before his death, none of which have been critically praised, and this one may well be the worst of the lot.  With an overemphasis on depicting the voodoo rituals at length, including an interminably long opening scene that doesn't make much sense until midway through the movie, there's not much entertainment to be had here.  There are some intriguing scenes with Karloff as a scientist trying to harness the power of the mind, and coming up with a scientific explanation for the zombies on the island, but these aren't followed through on, and what we end up with isn't worth watching.