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 the measurement tests of maximum rotation and 18,5% at the opacity test. The high percentage of disapproval at the rotation tests shows that a significant parcel of the fleet on study falls in with an adulterated fuel supply system. The disapproval at the visual tests and at the opacity tests, totalizes 27%, they can be directly related with general maintenance state of the vehicles. The operational difficulties for the implementation of the proposed measurement methodology, defined by the National Council of Environment - CONAMA, are also presented and discussed.