After defining the parameters describing the quality of blasts in tunnel driving--the pull efficiency, the overbreak and the HCF factor (the ratio of thee observe length of half cast on the tunnel wall to the total drilled length of the contour holes) --the paper examines the influence of the quality of the rock mass on said parameters. As indicators of the quality of the rock, the RMR (Rock Mass Rating) according to Bieniawski and the rock mass strength according to Hoek and Brown criterion are assumed. On the basis of observations in more titan 15 stretches of tunnels in different geological conditions and excavated with different techniques, it has been possible to outline t~:e relationships between RMR and rock mass strength and the blasting efficiency, the overbreak and the HCF. It is concluded that the rock mass structure has a not negligible influence when a quality threshold has to be set for a tunnel blasting operation; the threshold values as suggested by observation are indicated. Moreover, it has been found that the use of sophisticated drilling systems, using expensive equipment, does not provide advantages in cases of rock masses of poor quality. Finally, it has been found that HCF is a sensitive indicator of the quality of blasting, and that the same factor increases in an evident manner as the rock mass quality improves.