Season 1 NBC , 1967 - 1968 Season 2 NBC , 1968 - 1969 Season 3 NBC , 1969 - 1970 Season 4 NBC , 1970 - 1971 Season 5 NBC , 1971 - 1972 Season 6 NBC , 1972 - 1973 Season 7 NBC , 1973 - 1974 Season 8 NBC , 1974 - 1993