Opacity inspections were realized at the exhaustion pipe of 200 Diesel heavy vehicles (bus and trucks) of the circulating fleet in Belo Horizonte, Minas Gerais, Brazil. The inspections were realized following the procedures of evaluation of the smoke indexes (opacity) of automotive Diesel cycle vehicles in use, by the free acceleration method, according to the Brazilian standard NBR 13037 and with what was established in the CONAMA Resolution No 251 of 1999. This methodology involves at first a visual inspection, to verify if the vehicle engine shows an irregular functioning, apparent emptying, violation of the sealing of supply pump, alterations in the exhausting system, in the air admission system, as well as emission of blue smoke, a rotation test, that is necessary to keep the engine mechanical integrity, and, finally, the opacity test of the exhaustion gas. From the analyzed sampling, the total percentage of disapproval was around 86%, where 8,5% at the visual inspection, 59% at