First a little etymology. The first confirmed use of 'metal' as an adjective to describe music appears in a review by Barry Gifford in an issue of Rolling Stone published in 1968.
"Nobody who's been listening to Mike Bloomfield—either talking or playing—in the last few years could have expected this. This is the new soul music, the synthesis of white blues and heavy metal rock."It described the band Electric Flag. In all honesty the use of the term in the 1970s by Creem Magazine to describe Sir Lord Baltimore was more akin to the contemporary sense. This leads me to believe that no radio show can claim to have begun before the mid 1970s and truly have any metal music to air. So it is no surprise that few programs can be authoritatively dated to even the early 1980s. What follows is an incomplete list of the earliest and longest running known metal programs in radio.
WERS "Nasty Habits " started in about 1980. The program released a Live CD in 2002 on their 21st anniversary. Reports indicate the program was canceled in early 2004. Matty O'Dette put together the comp. Charlie Andrews was host from 1988 to 1989. I think the first host was Travis Squier. I don't know who closed it out.
WVUA "The Metal Zone" debuted in 1981. It was hosted by Dave the Metal Guy for 27 years. It was canceled in 2008 by a 19-year-old freshman. Ouch.
KUSF "Radio Rampage" first aired in 1982. It's claims to continuing uninterrupted are somewhat disputed but it's still on air today. It's on 2-8AM Sunday morning in San Francisco.
WREK "Wreckage" has been on since 1984 in one form or another. The show airs every Friday evening at Midnight. More here.
WSUP "Heavy Metal Meltdown" began in 1983. it's run through a number of slots and hosts over the last 27 years but it's still going today.
WEOS "Metallic Onslaught" has been on since 1984 but airs irregularly. The old host was Joe Wyatt, I dont knwo who's there today. More here.WNYU "Hellhole" first aired in 1984. Hosts over the years included Jen Meola, Jackie Vendetti, and Eric Peletier.
WHJY "The Metal Zone" debuted in 1986. It was hosted by For 17 years by Mike Gonsalves Previous to that he had a metal show on college station WXIN. I know that's not exactly the same show but it's the same format in the same city and worth noting. He died in 2003.
KFAI "Root Of All Evil" started in 1985 and ran for 23 years ending only when host Earl Root died May 24, 2008 of cancer. More here.
WITR "Sudden Death Overtime" in Rochester had a long running show canceled in Dec 2009. It's start date is uncertain but current host Ron confirms that it's been running in one form or another since the early 90s.
WOXY "Massive Metal For The Masses" Pre 1984? This program ran on Sunday nights from 7:00 PM to 1:00 AM. It was hosted by Mr. K. It ran through at least 1986 but assumed to be much longer. I could use more concrete details here.
WNEW "Metalshop" A program in New York city hosted by Charlie Kendall. It began in January of 1984 and ran for 11 years. It was syndicated to other stations and only died when the station flipped to AAA in 1995.
KJHK in Lawrence KS, has had a metal program at least for 10 years named "Malicious Intent". WTSR "chainsaw symphony" has run over a decade. KLSU has had a program for approximately 20 years but it seems to have taken a few breaks. WCLH used to run "Metal Monday's that went on for years sometimes with full days dedicated to the genre.Razor ray was on WRBC in Lewiston, Maine for over 10 years but seems to be gone now. KPOO had a brutal metal program in the 1980s.. no corroboration on that yet. But I know that's not everyone. WVFS, WVBR, WKNH, WRHU, and WRAS, all had long running metal programs. And there were surely metal programs before 1980. As is often the case... we're one aircheck away from victory. Feel free to contribute if you know more.