Posters showing images of both activities, crossed through in red, were put up by Northamptonshire Police.
"It came to our attention that bushes by a picnic area and a lorry park at the service station were regularly being used by people as a toilet," a spokesman said.
The signs were designed to be clear to "people from across the world".
About 20 posters have been put up at the northbound service area "to encourage people to use the facilities inside the service station building".
Ch Insp Tom Thompson, who is leading the crackdown, said it was "perverse" that people would use the hedgerows when the services were free to use.
He said there had been a growing problem with the smell around the hedgerows, saying there had been "more and more complaints from people trying to picnic over the summer months".
If the signs are successful they may be used in other locations.
"Clearly by printing these posters we'll get a good debate going and hopefully people will start using the toilets," he said.
A spokesman for Roadchef, the company which runs the Watford Gap services, said: "Following a very small number of incidents, the police have introduced a sign in the car park... requesting that motorists use our newly-upgraded toilets inside the building, rather than foliage in the car park.
"We do not condone people using our site inappropriately in any way, and monitor car parks at all of our sites to ensure every customer receives a good experience when using our facilities."
Visitors to Watford Gap services on the M1 are being warned not to urinate against walls and defecate in bushes.