PERHAPS it is the distant unconscious memory of our prehistoric existence as cave people cooking around an open fire or simply a desire to stay outside in the warm sun -- whatever the cause, everyday millions of Americans prepare their meals on outdoor grills. In back yards, at campsites, on beaches, boats and high-rise apartment terraces, we are a nation of barbecue lovers.

Most of our barbecue devices are grills that apply direct heat to the food from below. This is a primitive method of cooking in which the food's natural juices are lost to the flames, but the process usually cooks away the excess fat at the same time.

My first charcoal grill consisted of a section of an old unpainted wrought-iron fence that I set above four cinder blocks. Wooden chips went on a bed of sand below the grill and everything worked very well. This straightforward approach is quite sufficient for an occasional outdoor cook who has a bit of cleared land for the blocks.

The first thing you will want to consider when selecting an outdoor grill is the type of heat source you want. There are three basic setups: solid fuel such as wood chips, charcoal or briquets; gas, either bottled or directly off your house line; and electrically heated lava rocks used in the Char-B-Que. Solid fuel has the advantage of being readily available, portable in most designs and the only real source of what we usually describe as "the barbecue taste." It can, however, be very messy, difficult to control and a number of medical authorities have commented on the possible danger of cancer-causing agents that flare up from fat drippings on hot charcoal.

Gas is clean, controllable, portable in the bottle form but less convenient to purchase. Electrically heated lava rocks are always ready to go as long as you are near a power outlet. I feel that the gas and electrical units are more convenient than solid fuel, but they lack the special flavoring that is associated with charcoal and wooden chips.

The units are divided into several arrangements based on fire box design: There are bowl shapes, trough shapes and box shapes. The bowl-shaped units look like deep woks. The heat source goes into the base and the grill is placed above. The trough shaped designs have the appearance of an oil drum cut in half. Once again, the heat is below and the grill is above. The box shapes look like suitcases with a grill pattern on the top side.

Whichever heat source or fire box shape you select, it is important to make sure that the distance between the top of the heat source and the bottom of the food can be varied without difficulty while the food is being cooked. The best way to grill foods requires that you cook the ingredients until almost done and then lower them to the heat source for final "crusting." This is almost impossible without a system for raising and lowering the grill. t

In addition to the variable grill distance mechanism, it's important to select a grill made of a material that will stand up to the outdoors. Make sure that the manufacturer has guaranteed that the fire box will not rust. The best material is enamel-covered sheet steel or protected cast iron. If you are about to purchase a portable unit and you expect to be moving it about while it is still hot, make sure the handles are firmly attached with more than one screw or rivet per side. Check to see that there is sufficient room between the handle and the outside wall of the unit for you to get a heavily gloved hand between the two without touching the wall.

Be very careful to avoid shabby construction in the leg and wheel support system. Wheels should be held in place with wing bolts or nuts that thread onto the axle. Cheap clip-on caps can slide off under the weight of a full grill, scattering your ingredients and burning fuel. Make certain that the method of joining the crossbars of the grill to its outside frame is strong and secure. The grill iron is subjected to considerable weight and enormous heat, so bad welding of its joints is dangerous. Outdoor grills are serious pieces of cooking equipment that are used in complex and sometimes difficult environments. Personal safety demands that only products with superior construction be used.

I also suggest that you purchase a unit that comes with a tightly fitting hood. Anyone who has mastered the art of the "Texas" barbecue or smokehouse cooking knows that the cover is essential.