To identify the availability of some essential drugs (ED) at primary health care units of the Mexican Ministry of Health (SSA), using data from a research work conducted in 1996 and 1997. A random sample of the 18 sanitary jurisdictions of states participating in the Extension of Coverage Program (ECP) was drawn. All primary care units from selected jurisdictions were evaluated through an inspection visit, using a checklist that included 36 ED. In phase I of the analysis, the absolute number and proportion of units without a single item of any of the 36 drugs were calculated. In the units with available drugs, the median of the distribution was also calculated. In phase II, the medians according to the type of health unit and state was obtained. The statistic utilized for the comparison of the medians was the Scheffé test with one way variance analysis. In addition, the drugs were classified according to their therapeutic indication and the medians and proportion of available drugs were calculated. The differences in proportion were evaluated with the statistic chi 2. During the visits, on average, 18 of the 36 drugs included in the list of the study were found in the health units. The availability of antibiotics, antituberculosis drugs, and antimalarial drugs was particularly poor. In contrast, oral rehydration salts, family planning methods and vaccines were usually available. In general, the PAC3 states presented the best availability figures. The Ministry of Health of México will have to develop an enormous effort to overcome the obstacles related to the supply of essential drugs in primary health care units. Otherwise, all other efforts directed to meet the needs of the non-insured population will end up being useless, and enormous amounts of the already scarce resources of the health sector will be wasted since drugs are a vital component of the long chain of health care.