I'm not really sure about the saddest moment in the series. I'm thinking of Kowalski's poem to Doris on the episode Hot Ice.

I'm going to go with...when Skipper got his mind-jacked and almost drowned in the Blowhole special. That was really depressing for a kids show!

^ Good point. for a kids show, it almost reminded me of the episode of "Family Guy" where Stewie kills Lois (or thinks he did)

I would say when Private, Kowalski, and Rico hugged when they thought Blowhole was going to shoot them with the diobologizer. I literally said "Awww!" and was about to cry until Julien came. :'(

For me in was in 'Snakehead' when we are led to believe that Skipper had been eaten.

When blowhole tells 'young private' skipper was not coming to the rescue...

I can't say for myself, but when Skipper thinks Private is dead, that was pretty sad for him.

Maybe when the Vesuvious twins were abusing all the animals in Pet Peeved? It was a pretty dark moment, but ot could also be when Kowalski, Rico, and Private were about to ne turned evil and were hugging. That was a pretty sad moment.