The award-winning actor, who plays the title role in the ITV drama, said the fifth series is being written, but the sixth season has not yet been confirmed.
"We've talked about a sixth series - they talk quite firmly about it but we'll see," he said.
"It'd be awful if it just got bad because we're flogging it out and churning things, but we do it every other year because it takes so long to get written so we can keep it fresh."
Martin wouldn't like a happy ending for the grumpy Doc and Louisa, who's expecting his child.
"I'd like to see him being a father - I think he would be awful and there might be some fun there," he said. "I think we're going to experiment with them cohabiting with an infant in tow, which is going to be disastrous."
He added: "I like to see him fail and be punished. If things are going too well, I'll walk him into a door frame or something just to punish him."
The huge success of the show - the fourth series has been released in DVD - is a huge surprise to him.
"We hoped people would like it but it's been gratifyingly successful. It's all been a lovely surprise," he said.
:: Doc Martin series 4 is available on DVD now.