Researchers found that rats injected with stem cells 30 minutes after a stroke had almost normal brain function restored within a fortnight.
The Bolivian research team said the method has potential in human trials, 'BBC News' reported.
Current practices involve treating patients with "clot-busting" drugs in the "golden hour" after a stroke has taken place.
The study adds to other research which found that stem cells could aid stroke patients by boosting the body's ability to repair tissue damage.
Stem cells are the body's "master cells", with the potential to become many different cell types, and theoretically replace cells lost through disease or injury.
Recent tests in humans have shown some promise, with stroke symptoms improving after an infusion of stem cells.
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Researchers from La Paz University Hospital, extracted a certain type of stem cells from fat and bone marrow, then injected them into the blood vessels of rats shortly after they had suffered an artificially-induced stroke.
Even though the introduced cells did not appear to travel to the affected region of the brain, the rats still did better than other rats who did not receive the cells.
Within 24 hours, they were already showing a speedier recovery, and two weeks later, they registered almost normal scores on behavioural tests.
Early introduction of the cells might even interrupt the typical "chain reaction" of tissue damage which follows a stroke, in which the initial injury harms additional cells in surrounding areas, researchers said.
"Improved recovery was seen regardless of origin of the stem cells, which may increase the usefulness of this treatment in human trials," Dr Exuperio Diez-Tejedor, who led the study, said.
"Adipose (fat) -derived cells in particular are abundant and easy to collect without invasive surgery," researchers said.
Researchers suggested that it might be possible to overcome the risk of immune rejection of the donor cells in humans.
However, a spokesman for the Stroke Association in UK said that human trials of this particular technique would not be possible in the near future.