WHOEVER declared that nostalgia isn't what it used to be may be further inspired by a series of madcap commercials that seek to stimulate fond memories of sales spiels that never ran, for a product that never existed.

Nick at Nite's TV Land, a cable television network owned by Viacom Inc. that presents reruns of programs like ''The Ed Sullivan Show'' and ''That Girl,'' is augmenting its ''retromercials'' -- revivals of actual old commercials -- with a dozen promotional spots for an imaginary brand, Twip. Those tongue-in-cheek spots for the fictitious flagship of the equally fictitious Twip Chemical Corporation wittily dissect the methods that Madison Avenue has used to peddle prosaic consumer packaged goods since television took off.

''After retromercials, we started talking about some form of retro parodies of commercials,'' said Thomas Hill, creative director for TV Land in New York.

''The idea was to focus on the advertising as advertising,'' he added, ''by having a product with random properties, so you can do anything with it you want to.''