The boy who turned into a dog!
It was brilliant, based on a book of the same name by Allan Ahlberg. I seem to remember Susan George played his mum? Later series with different dogs were never quite as good.
In general, there would be a moment in every few episodes where Eric (dog-boy!) discovering his changing into a dog is caused by adrenaline, and he would meet with an adult (played by Anthony Stewart Head) who has the same problem! This was before mr Head was in Buffy the vampire slayer!
There have been 4 dogs used over the years. Pippin was the first, who was the offspring of one of the dogs who played Benji.