SAN FRANCISCO – Presiding over a company with a market value of $143 billion apparently gives Silicon Valley’s most famous billionaires a good sense of humor – and a case of corporate potty mouth.

Senior executives at Google Inc. launched their annual April Fool’s Day prank Sunday, posting a link on the company’s home page to a site offering consumers free high-speed wireless Internet through their home plumbing systems.

Code-named “Dark Porcelain,” Google said its “Toilet Internet Service Provider” (TiSP) works with Microsoft Corp.’s new Windows Vista operating system. But sorry – septic tanks are incompatible with the system’s requirements.

The gag included a mock press release quoting Google co-founder and president Larry Page, a step-by-step online installation manual, and a scatological selection of Frequently Asked Questions. On some Google sites, the company’s official logo – a multicolored “Google” that changes according to the season and on holidays – substituted a commode for the second “g.”

“There’s actually a thriving little underground community that’s been studying this exact solution for a long time,” Page said in the facetious statement. “And today our Toilet ISP team is pleased to be leading the way through the sewers, up out of your toilet and – splat – right onto your PC.”

Marissa Mayer, a Google vice president, called TiSP a “breakthrough product, particularly for those users who, like Larry himself, do much of their best thinking in the bathroom.”

TiSP is the latest April Fool’s joke at the Mountain View, Calif.-based company, where hijinks pervade cubicles all year long. In blogs, Google employees joke about the recent injection of green dye into milk in the cafeteria, while another talks about zany underlings filling the vice president of engineering’s office with sand.

Eric Raymond, a software developer in Malvern, Pa., and author of the New Hacker’s Dictionary, said TiSP nailed several important tenets of hacker humor.

The concept of free wireless access parallels a legitimate, four-year deal between Google and EarthLink Inc. to provide free wireless Internet service throughout San Francisco starting in early 2008.

As part of the spoof, Google said TiSP would be offered in three speeds: Trickle, The No. 2, and Royal Flush.

“The leitmotif of hacker humor is precise reasoning from utterly bizarre premises, and once you’re in that groove, you’re absolutely fearless about going deeper,” Raymond said. “We also have a tendency to deliberately zigzag between highly intellectual humor and utter slapstick. The more zigzags you can manage in a single spoof, the funnier it is.”