Scotland's blood banks have less than a three-day supply of a universal blood group which can be given to anyone in an emergency.
The Scottish National Blood Transfusion Service (SNBTS) said its supply of the O Negative type is below half the "ideal" six-day stock it aims to have of all eight blood groups.
O Negative can be given to anyone in an emergency, and is the only safe option when a patient's blood group is not immediately available to medical staff.
It can also be given during specialised procedures for unborn babies.
The service is urging donors of all groups, particularly O, A and B, to give blood ahead of the October school holidays, to allow it to build supplies.
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."
