All the Lassies were actually male dogs because female collies tend to "blow coat" (go through a massive hormone-induced shedding process) with each heat cycle. While males blow coat as well in reaction to a change in season, it is much less noticeable than what occurs with an intact female. By the time that spaying, which would reduce the dramatic shedding of the female, became commonplace, it had become tradition to use a male in the role. Additionally, it was believed that males, who often outweigh their female counterparts by as much as fifteen pounds, would look more impressive on film.
This was one of the first long-running television shows to remain in its time slot and day of the week during its entire prime-time run on a US television network. It became a syndicated program for the last three years of its run.
Pal, the first ever dog to play the legendary dog "Lassie" in Lassie Come Home (1943), appeared in the pilot of this TV series (broadcast as the first two episodes). It was his last appearance before his death in 1958.
The famous "whistle" theme associated with the show's opening and closing credits was not actually introduced until Season 5. The previous four seasons used a more traditional orchestral theme for its opening and closing music.
The original Lassie TV show starred the Miller family with Tommy Rettig as Jeff, Jan Clayton as his Mother, Ellen, and George Cleveland as Gramps. The first two seasons of Lassie with the above cast garnered an Emmy award each of those years for best prime time children's show. Lassie never won an Emmy after that. Tommy Rettig was getting too old for the part, so he voluntarily left and was replaced by Timmy and his adoptive parents, the Martins. The critical quality of the show (writing, acting, etc.) began to decline in the third season and suffered considerably when Timmy, the Martins and new producers took over. With the notable exception of a couple of acclaimed Christmas shows the popularity of the dog Lassie and her acting abilities carried the show for many years. She was played by a boy dog called Pal Jr. who was the son of Pal and looked almost identical to the original Pal from the MGM movies and TV pilot show. Lassie's owner and trainer, Rudd Weatherwax, and his two dogs, Pal and Pal Jr., were phenomenal. Their acting abilities as a team were what made the Lassie brand what it was and still is today.
Jon Provost left the series after seven seasons because he tired of playing the role of Timmy. This made it necessary to fire the rest of the human cast. The cast speculated that either Provost or new star Robert Bray wanted too much money. But Provost did not want to continue playing a naive child into his teenage years.
Tommy Rettig wanted to leave the series after three seasons. He was fifteen and did not want to continue playing the role of a child. At the same time, Jan Clayton also wanted to return to musical theatre. Timmy was added to the cast to start a slow transition. But when George Cleveland suddenly died, the change to a new cast was accelerated.
The collie that played Lassie the longest was named Baby. Baby was the grandson of Pal, the original Lassie. Baby played the role for six years until he suddenly died at the age of eight. He is the only Lassie not to live to the age of seventeen.