Animal enclosures are designed to locate the subject without using restraint so the animals do not suffer from restraint stress and confound the results of the startle testing. The animal is free to turn around and make other movements. The SR-LAB enclosures are configured to focus the source of stimuli on the animal’s center line assuring the level of the stimuli is consistent. The SR-LAB cylindrical animal enclosure monitors animal movements with a closely coupled accelerometer sensor. The tubular design of the animal enclosure ensures that the animal remains centered over the sensor for consistently reliable results. Other systems with a square animal enclosure permit the animal to be off-center which affects the consistency of the response. The enclosures are made of transparent acrylic for easy cleaning and convenient observation. All enclosures are adjustable in length and come in five different sizes to accommodate adult and juvenile rats and mice— Small, Small/Medium, Medium, Large and X-Large. Also available are slotted enclosures for tethered animals in the same sizes except Small.