OBJECTIVE To present the conditions that favour or limit the integration of mental health into health centres, based on the perceptions of health workers and on observations made by researchers. DESIGN A study was conducted between April 2012 and February 2014 using a non-participant observation technique plus interviews with health professionals. SETTING Descriptive exploratory study conducted in 19 health centres in Mexico City. METHOD The selection of centres and participants was intentional, followed by the snowball technique in order to reach data saturation. Two guides were use, one for collecting information during the observation and the other one for interviews. The observations were registered in field notes, while the interviews were audio recorded. All collected information was stored in Word files. The analysis of field notes consisted of three levels of reading, and the interview analysis was based on "categorisation of meanings" proposed by Kvale (1996). RESULTS The aspects that favour or limit the integration of mental health services involve three broad categories: a) programs and methods that organise services, b) infrastructure and material resources and, c) human and information resources. CONCLUSIONS Actions targeted at including mental health into productivity reports and into already established goals, would contribute to the integration of mental health care, as well as promoting the idea that mental health is part of overall health, and to increase the public investment in health.