From a public health perspective, Mexican Americans are widely recognized as an understudied population. Available evidence suggests that Mexican Americans shared equally with non-Hispanic Caucasians (Anglo Americans or "Anglos") in the national decline in coronary heart disease (CHD) mortality during 1970-1976. This finding is somewhat unexpected in view of Mexican Americans' relatively lower socioeconomic status, and it suggests that studying this ethnic group may be strategic in understanding which factors have operated to produce the decline in general. Data obtained from a random sample of Mexican Americans and Anglos residing in three socioeconomically and culturally distinct neighborhoods in San Antonio, Texas, indicated that Anglos were significantly more informed than Mexican Americans about CHD prevention and generally reported engaging in more behaviors specifically aimed toward prevention than did Mexican Americans. Levels of knowledge were not particularly high in either ethnic group; moreover, reported behavior lagged considerably behind knowledge. While Mexican Americans were significantly less likely than Anglos to volunteer knowledge about major heart attack symptoms, there were negligible ethnic differences in awareness of the need for prompt utilization of medical treatment resources when a heart attack actually occurs. Awareness of the need for prompt action was notably high among both ethnic groups. Although limitations in the nature and scope of the data dictate that these findings should be interpreted with caution, the results tend to argue in the direction that prompt access to medical treatment when coronary emergencies occur may have made a relatively greater contribution to the CHD mortality decline in both Mexican Americans and Anglos than did changes in life-style. Much additional data are needed, however, before a definitive explanation of the decline will be possible.