Algernon, the rodent half of "Charlie and Algernon's starring duo, has made good use of his three-month layoff. In the new and slightly more elaborate edition of the show that opened at the Eisenhower Theater last night, little Al stillscampers down co-star P.J. Benjamin's arm, still wiggles his tail winningly, and still shows a flair for following a mouse-sized spotlight across the stage. (Or could it be the other way around?)

But Al has added at least one new routine to his repertory -- climbing onto Benjamin's shoe and ascending his pants leg. By the time the show gets to Broadway a few weeks hence, this mouse could be stiff competition forNadia Comaneci.

"Charlie and Algernon's" human participants have been busy, too since the show ended its spring engagement at the Terrace. The results of their work are less emphatically successful. In essence, they have the same showon their hands with the same considerable virtues and the same nagging weaknesses.

For those who missed it the firsttime around, this is the story of Algernon, an ordinary mouse, white in color, who is the first successful subject of aneurosurgical procedure for multiplying intelligence; and Charlie, a human being, IQ 66, who yearns to be normal and accepted -- which means smart (as even the dumb Charlie has concluded).

The exact nature of the operation that turns this pair into a man and mouse in a million is a trifle hazy -- something about "removing the damaged portion of the brainand replacing it with healthy tissue that has been chemically revitalized." It sounds like something developed by Prof.Irwin Corey in collaboration with Rex Morgan, MD. Nevertheless, Charlie follows Algernon onto the operating table, andis soon reading "War and Peace" overnight and studying eight languages simultaneously.

Despite considerable rewriting, Charlie's retarded phase is still the repository of the show's worst problems -- and they still have to do, disproportionately, with David Rogers' book. The doctors, while better motivated now, are still rather one-dimensionally ogre-ish. When informed that Charlie can't remember his parents, the more sinister of the medical men, Dr. Strauss, replies: "Very well, we may assume they're dead," which seems a bit precipitious as well as harsh. And the pre-genius Charlie remains far too given to ponderously ironic lines designed to prove there is wisdom in stupidity.

Rogers has definitely cleaned up one previously murky area of the plot. Charlie'sbakery co-workers no longer turn against him inexplicably.Now we understand he has been an object of ridicule right along, which helps explain his enthusiasm for super-surgery.Still, "Charlie's dialogue and plotting are too often unacceptably dumb,ing are too often unacceptably dumb, even for science-fantasy, and its message -- it has one, all right -- -is too often dealt out in pontification. All in all, the book would be well-served by a shot of the same medicine administered to its two principal characters.

Yet when the show comes to life, it is with a bang -- and thanks to the same writer, who is the lyricist as well. "Reading," the song in which Charlie summarizes the plots of "Robinson Crusoe," "Dr. Jekyll and Mr. Hyde" and "War and Peace," is a truly stunning patter song of a kind that few musicals attempt any more (and Benjamin, a sensationally effective performer here,brings down the house with it). Not only is the number an accurate account of all three novels, but Rogers' lyrics chronicle Charlie's burgeoning genius as words like "epitome" and "evolve" creep gradually into his vocabulary.

The other knockout number is the title song, with its vaudeville intervals of mouse-related gags (like the one about the man whois on his way to a psychiatrists' convention and stops a mouse to ask for directions: "If you're talking to a mouse, you're halfway there," comes the reply) and its man-and-mouse dance. Otherwise, the score, like the show itself, is modest and highly agreeable.

The show may have lost a certain ephemeral something with the transfer to a larger theater, but its admirers can put their worst fears to rest. The producers have not felt the need to expand their 11-member cast, or hype the sound coming from their small orchestra, or install body microphones in the costumes, or hire a dog to play the mouse. "Charlie" is still a small musical that packs a solid punch.