Until now, experts had viewed such stress as relatively benign.

The new findings by ethologist Jonathan Balcombe of the Physicians Committee for Responsible Medicine are published in Contemporary Topics in Laboratory Animal Science.

The findings say a mouse which is picked up and briefly held experiences several physiological reactions. As stress-response hormones flood the bloodstream, the mouse exhibits a racing pulse and a spike in blood pressure.

In rats and mice, the growth of tumors is strongly influenced by how much the animals are handled, the study says.

Until now, humane concerns focused mainly on the experiments themselves. The new findings suggest that routine procedures, such as blood draws and use of stomach tubes, are terrifying for animals.

"In essence, there is no such thing as a humane animal experiment," says Balcombe. "Fear or panic ensues when the animal is touched or stuck with a needle."