[Venturer]

The nutcase that left Bonanza because he was afraid of being typecast , Pernell Roberts. Didn’t see much of him afterward.

[Jonty30]

Trapper John MD.

[Venturer]

Then you have guys like James Arness who rode Gunsmoke right into old age and then made a couple of Gunsmoke movies.

[Las Vegas Ron]

Didn’t see much of him afterward.

He went on to do Trapper John MD which had a good run for 7 years.

According to wiki, he had guest appearences in 60 different shows.

[Future Snake Eater]

Pernell Roberts starred in the pilot for "San Francisco International" and was then replaced by Lloyd Bridges when it went to series.

[DManA]

I was just a kid when that happened but I get the impression that a lot of viewers really resented it when he left. People loved their Bonanza and they didn’t like it when big shot actors messed with it.

[Tanniker Smith]

Pernell Roberts had a decent run on “Trapper John, MD”, based on an older current day version of Wayne Rogers character.