If Memory Serves is the thirteenth episode of season three of Dexter's Laboratory.
At the beginning, Dexter and his family are looking over photos that were taken of trip they took to Hawaii, and as they're reminiscing, Dad notes how he finds it funny that people generally become more forgetful as they get older, which gets him and Mom to start remembering all the times that they've forgotten their house and car keys, the time Dad left for work the previous day without his pants on, and Mom notes how she wrote 1986 on her checks, and then she and Dad can't remember what happened back in 1986.
While hanging out in the lab, Dexter thinks of his parents not being able to remember stuff as stupidity, but Dee Dee comes to their defense by pointing out that they're not stupid just because they can't remember stuff--they just can't remember that far back. Dexter finds this inexcusable, but then he realizes that he can't remember anything from when he was a baby/toddler. So he decide to build a manchine to help him remember his babyhood. Then Dee Dee comes in and starts doing things she usually does, like the fanciful unicorn dance or going "Ooo, what does this but-ton do?" so he kicks her out and builds a memory storage device and starts uploading his early memories to it.
Then Mandark crashes through the ceiling and trys to steal the device. So Mandark and Dexter start fighting, with Mandark winning. He thinks that all the memories in the device are important scientific ones, but they turn out to be sentimental memories from Dexter's very early childhood, and most of it is nonsense (to Mandark)--one year for Halloween, Dexter dressed up as a scientist (looking identical to what he wears now) and when he first started wearing "big boy" underwear.
So Mandark runs screaming from the lab at all the nonsense and leaves. Dexter gets up and calls him a baby and the episode ends.