EXCLUSIVE: Inside Robert Duvall's Dying Regret — Never Being Called 'Dad'

Godfather star Robert Duvall had one terrible regret on his deathbed – he never fathered a child to carry on his name and legacy, despite having been married four times, RadarOnline.com can reveal.

The Lonesome Dove star, who passed away Feb. 15 at the age of 95, always felt as if he had one thing missing from his otherwise full life – his own offspring, sources said.

And it wasn't for a lack of trying, says a snitch. Duvall "always assumed he was the one with the problem," not fourth wife Luciana Pedraza, 54, or ex-spouses Barbara Benjamin, Gail Youngs or Sharon Brophy.

In fact, the Apocalypse Now actor was so desperate for an heir he and Luciana even tried in vitro fertilization just five years before his death, a source dishes. At the time, he was in his 90s and the Argentinian native, who is 40 years his junior, was pushing 50.

"I guess I'm shooting blanks," he said in April 2007, two years after marrying Pedraza, who was just 24 when they first met and started dating. "[I've tried] with a lot of different women, in and out of marriage. I thought of adoption, but [my wife and I] haven't yet."

He also had reportedly considered using a surrogate. It was especially painful because Duvall won an Oscar for 1983's Tender Mercies playing Mac Sledge, a washed-up country star who finds new happiness when he bonds with the young son of a widow.

The actor, who made his big-screen debut in 1962 playing Boo Radley in To Kill a Mockingbird, became a stepdad to first wife Benjamin's then 6- and 7-year-old daughters, Nancy and Susan, when they wed in 1964.

But he never adopted them during their 11-year marriage.