A BE58 PILOT EXPERIENCED SMOKE IN THE COCKPIT FROM THE JANITROL HEATER. HE SHUT DOWN THE HEATER; DECLARED AN EMERGENCY; AND RETURNED TO LAND AT HIS DEPARTURE AIRPORT.

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PRIOR TO DEP OUT OF ZZZ; I OBTAINED A CLRNC ON THE GND WITH APCH USING THE DIRECT PHONE LINE NUMBER. AFTER CLRNC WAS RECEIVED; I WAS IMMEDIATELY RELEASED AND DEPARTED FROM RWY X. MY CLRNC WAS TO PROCEED DIRECT TO OUR DEST OF ZZZ1 AND CLB AND MAINTAIN 4000 FT. UPON REACHING A SAFE ALT; I TURNED AS CLRED; AND CONTINUED CLBING TO MY ASSIGNED ALT. ONCE I CLBED THROUGH APPROX 2500 FT; I CONTACTED APCH. APCH INDICATED THAT I WAS IN RADAR CONTACT AND TO CLB AND MAINTAIN 7000 FT; OUR REQUESTED FINAL ALT. IT WAS AT THIS TIME THAT MY PAX (OCCUPYING THE COPLT SEAT) AND I SIMULTANEOUSLY NOTICED AN 'OFF' ODOR COMING FROM THE FRONT OF THE ACFT. IMMEDIATELY AFTER SMELLING THE ODOR I NOTICED A SMALL AMOUNT OF WHITE SMOKE RADIATING FROM UNDER THE INST PANEL. I IMMEDIATELY TURNED THE HEATER OFF. REALIZING THAT I WOULD NEED TO IMMEDIATELY RETURN TO ZZZ; I CONTACTED APCH; DECLARED AN EMER; INDICATED THAT I NEEDED TO RETURN TO ZZZ WITH SMOKE IN THE COCKPIT. APCH IMMEDIATELY CLRED ME BACK DIRECT TO ZZZ AND CLRED ME FOR A DSCNT. AT THIS POINT THE SMOKE THAT WAS IN THE COCKPIT HAD ALMOST TOTALLY DISSIPATED. REALIZING THAT I WOULD NOT GET THE VISUAL; APCH CLRED ME DOWN TO 1700 FT. I BROKE OUT OF THE OVCST DIRECTLY OVER THE FIELD AT 1700 FT AND WAS CLRED FOR A VISUAL APCH TO THE ARPT. I LANDED AND TAXIED TO THE HANGAR WITHOUT FURTHER INCIDENT. AN EXTERIOR VISUAL INSPECTION OF THE HEATER SHOWED NO OBVIOUS PHYSICAL DAMAGE. THE MECH LATER INDICATED THAT THE JANITROL HEATER INSTALLED ON THE ACFT WAS PRODUCING SMOKE FOR UNKNOWN REASONS AND WOULD NEED TO BE REBUILT. I FELT THAT DECLARING AN EMER IN THIS SITUATION WAS APPROPRIATE. PLTS ARE TRAINED THAT INCAPACITATION CAN HAPPEN QUICKLY WHEN SMOKE IS IN THE COCKPIT; AND TO TREAT IT AS AN EMER. I WAS FORTUNATE THAT THE SMOKE THAT WE SAW IN THE COCKPIT WAS CREATED BY A HEATER MALFUNCTION AND NOT FROM ANOTHER SOURCE THAT WOULD HAVE CONTINUED TO BURN. I THINK THAT IMMEDIATE ACTION WAS NECESSARY AND THAT MY DECISION WAS APPROPRIATE. SINCE THERE WAS NO FIRE; I DETERMINED THAT THE EVENT DID NOT QUALIFY UNDER NTSB 830.5(A)(4) FOR IMMEDIATE NOTIFICATION TO THE NTSB.