Southern Ohio evangelist Jim Brown claims that he and cohort Greg Hudson have discovered that the theme song to the old ''Mr. Ed'' television show contains a satanic message when played backward.

''A Horse Is a Horse'' contains the backward messages ''the source is Satan'' and ''someone heard this song for Satan,'' Brown claimed recently during a seminar he held for teenagers about rock and country lyrics in Ironton, Ohio.

Afterward, the inspired teens burned more than 300 albums, though

''Television`s Greatest Hits,'' which includes ''A Horse Is a Horse,'' was not among them.

Brown and Hudson are based in South Point, Ohio, and hold religious seminars devoted to topics other than lyrics. They also have a Christian popular music show on a small college radio station and said they have been researching backward messages for four years.

However, they couldn`t explain how anyone could hear a message, satanic or otherwise, when it`s recorded backward and played forward.

''We had a copy of (`Television`s Greatest Hits`) and were just curious to see if there were any messages at all,'' Brown said. ''At first we didn`t pick up nothing at all. Then sure enough, we heard the message.''

''Satan can be an influence whether they know it or not,'' he said. ''We don`t think they did it on purpose and we`re not getting down on `Mr. Ed.`

That would be like kicking baseball out of America.''

Yes, indeed. Just ask James Burnett, founder of the international Mr. Ed Fan Club, based in Cedar Hill, Tex. Burnett is galloping mad.

He publishes The Horse`s Mouth, a more or less monthly newsletter devoted to the show that was a staple of American television from 1961 to 1966.

''I am completely astounded. We have Khadafy bombing people in airports. We have starving and homeless people right here in America and this so-called Christian chooses to lambaste Mr. Ed,'' said Burnett. ''Wake up and smell the road apples, Rev. Brown.''