Jane Fonda: Good Genes or Good ... Spandex?

Jane Fonda is no "Monster-in-Law."

Here's the 48-year-old aerobics wonder back on the cover of one of her VHS workout tapes in 1986 (left) -- and 25 years later, the 73-year-old stretching it out on one of her brand new DVDs (right).

Her body is working "9 to 5."