Ten swimmers set out this morning from St John’s Lock near Lechlade hoping to reach Putney by next Wednesday 26 August.
They are sponsored to raise over £200,000 for the Stem Cell Foundation.
The challenge is called the Thames Doggy because all ten are promising to do a doggy paddle rather than the breaststroke or butterfly.
Anyone walking up the Thames and wanting to know if the swimmers are due to appear can register for updates on the Thames Doggy website.
On Wednesday the last leg to Tower Bridge will be by boat with a large inflatable pink dog on board.