The objective of the work was to evaluate the effect of conditioning time (during 14 days of ageing under vacuum) and sex (heifers vs. bullocks) on physicochemical properties of two beef skeletal muscles. The research material was made by random samples of m. longissimus dorsi (lumbar region) – LL and m. semitendinosus – ST from carcass of bullocks (n=12) and heifers (n=12), at the age of 18 months of Polish Holstein-Friesian breed Black-and-White strain. The pH, specific electrical conductivity – EC (mS/cm), CIE L*a*b* values, shear force and drip loss were measured. There was demonstrated a significant influence of sex on pH, colour (a* and b* values) and natural drip loss from the LL and ST muscle. Over the 14 day conditioning period, tenderness improved significantly along with steady growth of specific electrical conductivity of the evaluated muscles. The meat of heifers aged for 14 days under vacuum was characterised by lower pH, higher specific electrical conductivity, greater natural drip loss as well as more of red colour (higher a*) and of yellow (higher b*) as compared to the young bulls meat.