The Ancients observed that at this time of year, the Dog Star ascends in the heavens along with the hot summer sun.

The season of heat and humidity came to be referred to as dog days. And the Dog Star's proper name, Sirius, gave rise to another summertime term: siriasis, or heatstroke.

Siriasis is an acute-- and potentially fatal-- medical emergency. Approximately 50 football players have died from it in the past 10 years. And among high school athletes, it is the second-leading cause of death after spinal injury.

Anyone undergoing serious athletic training in hot weather, or anyone supervising others, should watch for the malady's symptoms.

The warning signs are: piloerection (rising of the hair) on the chest and arms, chills, a throbbing pressure in the head, unsteadiness, nausea and hot, dry skin. Often the rectal temperature of people in this state can rise to 105 degrees.

Heat stroke, or heat prostration, may occur even in healthy individuals who work or play so intensely that their body heat cannot be dissipated fast enough.

Several common training procedures, often considered to be acceptable athletic customs, can help bring on heatstroke. Here are some do's and don'ts for the sunshine athlete.

*Acclimation to heat is a gradual process. A player's first workout in hot weather should not be an all-out session in the heat of the day. A properly conditioned athlete can perform in hot climate with few ill effects. But before acclimatization, such exertion may be intolerable or even fatal.

*Don't skimp on water during workouts. Athletes performing heavy exercise should be encouraged to drink, taking even more water than their thirst dictates. Performance declines when an athlete has lost 2 percent of body weight in sweat. For a 200-pound athlete, that is only four pounds. Losing 5 percent of body weight in perspiration puts an athlete in danger of collapse. An added water tip: It should be cool, not cold, and kept in open containers rather than squirt bottles. The bottles tend to limit the amount of water a player takes at a time.

*Salt tablets: While they are supposed to be a help in hot weather, they can cause a problem. It takes a considerable amount of water to dissolve them and allow the salt to be absorbed from the stomach. That water comes from the vascular system (other body fluids), which already is losing water through heavy sweating. This can cause nausea, vomiting and possible dehydration. The effect desired from salt tablets can be obtained by salting table foods a little more heavily during the training period. If still more salt is needed, add one tablespoon of salt to each gallon of water to be taken during an event or workout.

*Exercising in plastic suits with the idea of losing weight rapidly is a potentially hazardous practice. To be sure, weight can be lost rapidly this way, but fat is not being burned off any more rapidly-- the body's weight is simply being drained through perspiration. Fatal heatstroke has occurred in men exercising in this way, even when the temperature is as low as 80 degrees. Since the suits are impermeable to water, the body cannot be cooled in its natural way through the evaporation or perspiration. A runner wearing such a suit in hot weather can experience a nine-degree rise in body temperature per hour.

*The natural cooling of the body is similarly impeded when heavy clothing-- football equipment-- is worn. High humidity further takes the body's air conditioning by slowing the evaporation of moisture from the skin.

*Athletes training in the heat should quit if they experience nausea or feel faint. This is the body's way of saying there's something wrong. This is one of the warning signs on the way to physical illness-- it's telling you you're overdoing it.