Two Swiss regions, the cantons of Vaud and Fribourg (VD-FR) and the canton of Ticino (TI), take part in the international research project MONICA (MONItoring trends and determinants in CArdiovascular disease). In this framework, two surveys were respectively carried out in 1984-86 and in 1988/89 to determine the distribution of the major cardiovascular risk factors in the adult population. This article presents the outcome of the sampling procedure and participation for the second survey. A two-stage sampling procedure was performed by randomly drawing first communes (51 in VD-FR region; 25 in TI region) and then individuals out of the communal registry offices (3300 in VD-FR; 2000 in TI). In the two regions, 2011 and 1458 persons respectively attended the medical examination, leading to overall participation rates of 61% and 73%. As the sex and age distribution of the sample and of the participants is very close to that of the population, a major selection bias is unlikely. For the VD-FR region, the improvement in the participation (57% in the first survey) is due mainly to a better response from foreign nationals and older persons, whereas in the TI region the drop in recruitment (78% in the first survey) may reflect a saturation point reached by the population after several years of an intensive prevention programme against coronary heart disease initiated in 1984. In both regions, unmarried people represent the only population group in which the participation rate has decreased. On the basis of a questionnaire proposed to the non participants, unmarried status, sedentary leisure activities and a previous cholesterol check appear as factors significantly associated with a higher refusal rate.