Longtime listeners of WHFS-FM (99.1) wailed when the pioneering alternative rock station dumped the Green Day and Foo Fighters tunes in January and began broadcasting Spanish-language pop music. For WHFS's mega-corporate owner, Infinity Broadcasting, however, the switch was all about defibrillating some flatlining ratings.

And that's exactly what Infinity has done. Now known as "El Zol" (officially, WLZL-FM), the station emerged as one of the big winners in the quarterly audience ratings compiled by radio researcher Arbitron Co.

El Zol's mix of lively DJs and upbeat music (salsa, merengue, reggaeton and other danceable tunes from the Caribbean and Latin America) helped the station finish a solid fifth in the Washington area among the young listeners (ages 18-34) Infinity is seeking. El Zol, which snagged a 5 percent slice of this youth market, according to Arbitron, still lags behind such music powerhouses as "urban" stations WPGC-FM (95.5) with 9.4 percent and WKYS-FM (93.9) with 8.0 percent, and "contemporary hits" Hot 99.5 (7.2 percent) among this age group.

But the real news is how far El Zol has come: The station is now attracting twice the share of young listeners as it did last fall, when it was still WHFS.

Lesson: Green Day, no. Juanes, si.

"The plan is working famously," said Michael Hughes, the Infinity executive who oversaw the station's relaunch and is now its general manager. "What we had projected, and what we saw, is now becoming a reality. There was a tremendous audience that wasn't being served" by smaller, less powerful Spanish-language stations in the area.

The demise of WHFS has also been a boon to its closest competitor, rock station DC101 (WWDC-FM). With no rock left on 99.1, listeners have been moving up the dial to 101.1 in droves. DC101 moved from fifth to second among listeners ages 18-34, with an 8.8 percent share, during the April-June period. It also tied for fifth (up from a tie for 17th) among all stations in radio's moneyed middle segment, 25- to 54-year-olds. DC101's "Elliot in the Morning" program has even become a contender for this older audience, finishing fifth overall in the morning time slot, just one-tenth of a rating point behind the soon-to-depart-to-satellite-radio Howard Stern, heard on WJFK-FM (106.7).

Other radio ratings notes:

* Urban adult-contemporary station WHUR-FM (96.3) was the area's most popular station among the advertiser-coveted 25-54 audience this spring, the first time in recent memory that it carried the top spot.

* The Nationals baseball games on its flagship station, Z104 (WWZZ-FM, 104.1) this spring haven't exactly been a home run. Yes, the station has improved its ratings slightly among the 25-54 crowd (baseball's prime audience) during evening hours, but the actual number of listeners hasn't grown very much. To wit: Z104 attracted a cumulative audience of 69,700 people per week during the 7-to-midnight shift this spring, compared with 59,900 last spring without the Nationals. Z104 carries the bulk of Nationals games; sister station WFED (1050), airs games on the AM band.

* Mix 107.3's shift to a broader variety of pop music -- a hot new format in the radio industry known as "Jack" -- hasn't yet lit up listeners since it began in April. Mix (WRQX-FM) actually lost audience share from the preceding quarter, dropping from 4.5 percent to 3.9 percent in the 25-54 segment. The decline hit Mix's morning man Jack Diamond, whose program dropped from fourth to ninth among adult listeners.

* The post-election blahs continued at WMAL-AM (630), the news-and-talk station that features conservative yakkers Rush Limbaugh, Sean Hannity and Michael Graham. The station, owned by the Walt Disney Co., recorded some of its lowest numbers ever among adult listeners, finishing 20th among all stations. During election season last fall, WMAL was the region's 11th-rated station, but its audience is off 45 percent since then.