Rock 'n' roll has always loved inspired amateurs, but a band can retain its amateur status for only so long.

Rock fans will accept or even embrace a certain sloppiness on a debut album if it's accompanied by enough passion or innovation. As a band starts to play regularly, though, it becomes professional by definition, and it almost inevitably gets better at what it does. If it doesn't, its persistent sloppiness begins to sound like a pose, like uninspired amateurism.

For a band like the Ramones, the Clash or X, negotiating the transition from amateurism to professionalism can be the trickiest part of a career. These three bands have succeeded in the shift, but underground punk fanzines are stuffed with the names of their contemporaries who stumbled at this crossroads and fell by the wayside.

The Replacements: 'Pleased to Meet Me'

Minneapolis' Replacements are going through this painful evolution right now. Their new album, "Pleased to Meet Me" (Sire, 9 25557-1), not only examines the process even as it's happening but gives every indication that the band will emerge from it as one of the best rock 'n' roll acts in America.

After a long history of wildly erratic albums and even sloppier shows, the Replacements are finally facing up to the challenge of their own talent. They fired their increasingly unreliable guitarist Bob Stinson and headed down river to Memphis to make an album with producer Jim Dickinson, one of those shadowy rock 'n' roll figures who played on many of the classic Memphis records of the '60s and later joined Ry Cooder's band.

Memphis has always had a high tolerance for eccentrics, and Dickinson didn't try to clean up Paul Westerberg's uninhibited yowl or the band's careening attack. He merely insisted that they keep time and play in tune, and that has made all the difference in the world. Westerberg, the band's emaciated, drunken genius, has evolved into a brilliant songwriter, and for the first time his melodies and rhythms are given their due.

The payoff is obvious on the first single, "The Ledge," a haunting minor-key set piece about a teen-age suicide. The trio of Westerberg, bassist Tommy Stinson and drummer Chris Mars creates an expertly controlled nervousness that's perfect for the first-person story of a kid who's scared of the jump he's about to make but proud that he could make some kind of statement.

The Replacements practice an odd form of ambivalence: Their imposing tendencies don't cancel each other out but instead escalate into matching frenzied passions. On "The Ledge," for example, the sheer terror of imminent death comes through as strongly as the peculiar satisfaction in the gesture.

The same kind of passionate ambivalence marks the Replacements' transition from a sloppy underground band to a focused, well-known act. Westerberg's throat-ripping anger and pose-puncturing lyrics have to be fiercer than ever just to hold their own within the more purposeful music.

On the band's raucus self-portrait, "I Don't Know," Westerberg screams, "Should we give it up or should we give it hell?" and his bandmates answer, "I don't know." Westerberg aptly sums up their position: "One foot in the door and one foot in the gutter." That track's hard-hitting funk immediately gives way to the surprisingly Sinatra-like jazz of the next, as Westerberg confesses his "Nightclub Jitters."

The album's best song is "Alex Chilton," a tribute to the pop-rock legend who sang with the Box Tops and Big Star (with studio help from Dickinson). When Westerberg cries out, "I'm in love with that song," the joy in the melody and his voice captures the essence of any fan's love affair with music. Even this song is filled with irony, though, as Westerberg questions why Chilton can be so good and still so obscure.

It's a question that has often been asked about the Replacements. Unlike Chilton, who has often seemed to run from success, Westerberg finally seems willing to face up to it. This album could bring it a lot closer. Chilton himself plays guest guitar on the album's final song, "Can't Hardly Wait," an achingly honest road song. The glorious melody, reinforced by the Memphis Horns, belongs on everybody's radio.

Tom Petty: 'Let Me Up'

If the Replacements are a bunch of inspired amateurs in search of some discipline, Tom Petty & the Heartbreakers are superb pop craftsmen in search of some rough-edged spontaneity. Their new album, "Let Me Up (I've Had Enough)" (MCA 5836), is an attempt to get back to their roots as a Florida bar band that covered songs by the Stones and Faces.

Unfortunately, Petty's strength isn't gritty R&B. He's competent at it (he's certainly better at it than Little Steven or Lou Gramm), but his real strengths lie in his ringing guitar harmonies, his anthemic melodies and evocative lyrics -- the building blocks of folk-rock. When he tries to prove himself as a macho rocker, as he did on his disastrous 1982 album "Long After Dark," his melodies and stories tend to flatten out.

"Let Me Up" is too varied to be as bad as that, but it's not as pleasureable as Petty's 1985 reconciliation with his roots, "Southern Accents." Since then, Petty & the Heartbreakers (who headline at the Merriweather Post Pavilion July 20) have played a lot with Bob Dylan. Petty cowrote a song for Dylan's album last year, and Dylan has returned the favor by cowriting "Jammin' Me," the first single from Petty's new album.

It's not a classic Dylan song by any means -- the tune boasts a hard, catchy blues-rock riff, but the lyrics just throw together a bunch of phrases that sound profound ("Vanessa Redgrave," "Iranian torture") but don't add up to much of anything. The same is true of such Petty songs as "The Damage You've Done" and "A Self-Made Man," which don't say much beyond the titles and have no compensating melody. The one exception, "Think About Me," is a funny, sexy brag that could work as a Stones song.

Petty is at his best, though, when he drops the unconvincing tough-guy growl and just sings clearly and honestly. The album's three best songs find him confessing his romantic vulnerability with a wide-open voice and heartfelt melodies.

"All Mixed Up" admits to growing confusion about an old relationship, but the irresistible momentum of the horn-driven melody fills the song with optimism. In a similar manner, the lovely acoustic guitar arrangement of "It'll All Work Out" suggests the faith that Petty is searching for in the lyrics.

The album's best song, "Runaway Trains," is about a relationship that is beyond repair, but the synth-fed agitation of the verses gives way to the folk-rock acceptance of loss in the chorus. Rather than chasing some borrowed image of a rough-and-tumble rock 'n' roller, Petty should be devoting himself to the same kind of honesty he has achieved in this song.