This goes in my list of favorite theme songs of all time: Pyun Pyun Maru (ピュンピュン丸), a Japanese cartoon that aired in the late '60s/early '70s.
"Pyun Pyun Maru" was about a ninja boy and his little brother that works for a private detective that accepts any type of work they come. The show takes place in Samurai-era Japan but there is alot of anachronism in the stories, with the characters having working telephones and often seen watching TV (in the opening credits you can even see the TV antenna sticking out on the building). It was very heavy on slapstick and general cartoon silliness.
These days it's not that unusual for anime to have a "wacky", "zany" style. Heck, one studio, Gainax, runs on this. It was, however, pretty rare back in the '60s. "Pyun Pyun Maru" was Toei Animation's first attempt on this style, and it did very, very poor in ratings.
26 episodes were produced in the same production season, but they were broadcast years apart on NET-TV (a Japanese network now known at TV Asahi). The first 12 aired from July 3 to September 18 of 1967. The sponsor pulled the show, citing poor ratings and the fact that the stories were batshit insane. The remaining 14 remained dormant until it was dusted off the shelves and broadcast from December 29, 1969 to March 30, 1970. Throughout the 1970s to the 1990s the show was rerun, where it got better ratings there.
Unfortunately I've only seen very little of the show. A complete collection DVD was released in Japan few years back. If anyone could help me find it, I'd much appreciate it.
For the time being, here's the opening and closing credits. Don't bother asking for English translations; it wouldn't make as much sense as listening to it in Japanese.