The RSPCA received two calls about dogs locked in cars in Nottinghamshire over the hot Bank Holiday weekend.
Across England, the charity had 217 reports of dogs in cars between Saturday and Monday - as temperatures hit almost 30C.
In Nottinghamshire last year, a total of 100 calls were made to the RSPCA about animals in hot environments.
RSPCA campaign manager Holly Barber said: "Many people think it will be fine just to leave their pets for a minute or two but we know that this is all it takes for temperatures inside a car to soar to dangerous levels.
"Dogs die in hot cars - don't let your pet be one of the number.
"We have been doing a lot of campaigning lately about the dangers of leaving your dog in a hot car and it's really worrying that despite this some people are still putting their pets in dangerous situations.
"Our main advice to owners is not to leave dogs in cars and our main advice to passers-by is to call 999 if they're concerned about an animal in a hot environment."