The aircraft departed Buenos Aires (EZE) for a cargo flight to S?o Paulo-Viracopos (VCP) via Porto Alegre (POA). En route on the first leg, at an altitude of FL170, a crew member headed to the cargo compartment, and noticed the presence of smoke. he tried to extinguish the fire with emergency equipment, but did not succeed. Some 16 nm from Melo Airport (MLZ), Uruguay, the crew declared an in-flight emergency to Montevideo ACC. The operating hours of the only airport nearby, Melo, are from 07:00 hrs to 19:00 hrs, so it was closed. Montevideo ACC contacted the airport guard who switched on the runway lights and reported the emergency to fire fighters and police located in the city. The aircraft found the runway lights and landed on runway 07. Touchdown took place 340 m from the threshold. The F-27 stopped 640 m after touchdown. The crew evacuated through the cockpit windows, due to the intense heat and smoke in the cargo area. Ten minutes after landing, the fire fighters arrived and succeeded in extinguishing the fire. The airplane suffered considerable damage in the cargo compartments E and F. PROBABLE CAUSE (translated from Spanish): Numerous oil samples, plant tissues, and several animals that were transported, would have reacted on contact with an exothermic chemical reaction and associated combustion. The event originated with the spilling of liquid from containers not suitable for differential pressure, with little excess capacity for expansion and filled with indiscriminate materials. Low stringency in filling forms for description of product delivered for shipment in company offices and little control or careless handling of the load would have contributed to the occurrence of the accident.