Dr Moira Carter, from SNBTS, said: "Although we are currently able to meet hospital demand we need to give our stock levels a significant boost before we enter the holiday period.
"This year we are particularly concerned as we expect that more families will head off for some much-needed sunshine given the poor summer.
"When that happens we find that donor numbers fall by up to 20%.
"Specifically we are asking for all group O, A and B donors to make sure that they give blood in the coming days and weeks."
SNBTS director Mary Morgan said: "Blood groups O and A are the two most common, therefore they are the most needed by Scotland's patients. O Negative donors are uniquely placed to help patients when they need it most as their blood group can be given to anyone in an emergency."