Over a period of 388 days, associations were sought between the occurrence of acute myocardial infarction (AMI), levels of atmospheric nitrogen-containing gases (NO, NO2, NH3), temperature and relative humidity. During this period, there were 282 AMI validated by the WHO-MONICA criteria. Analysis of the data revealed, a decrease in the area, in AMI, when daily nitric oxide levels were above 13 microg m(-3) and when the mean daily temperature was below 13 degrees C.