My seven year old rat terrier has been house broken for many years, but lately she's been peeing while laying down sleeping. Not just a little dribble but a copious amount.
At first I was fussing at her about it, but now I'm thinking that perhaps she can't help it?
She's been taking prednisone and enalapril daily for other problems. Could it be the prednisone causing the bladder to relax?

Hi gutapa.
I have the same problem with one of my mini poodles.
She is unable to control her bladder when she is sleeping.

There are drugs available that help but apparently they have side effects.

There is also a surgery that can be done in some cases (depending on the cause of the urination) but it is not 100% and might not work anyway.

It could also just be a simple urinary tract infection.

Take your terrier to the vet to determin the problem, a lot of them can be fixed :)