In their wild state, Guinea pigs are found on grassy plains. They move together in small groups (herds) eating grass or whatever other plants they come across. They are most active during dawn and dusk, when it is harder for predators to spot them. If startled they can run for cover with surprising speed. Domesticated Guinea pigs have developed a different rhythm, and have longer periods of activity followed by short periods of sleep in between. Activity is scattered randomly over the 24 hours of the day.

Unlike many rodents such as mice, rats, or squirrels, Guinea pigs are not very athletic. Jumping gaps, climbing, and fearlessness in the face of heights were not skills Guinea pigs needed in their environment, although they can hop over small obstacles with agility. When excited, Guinea pigs tend to do little jumps in the air (known as "popcorning"). Even so, they can perform feats such as climbing stairs, of certain heights, if prompted.

Guinea pigs are vocal animals, with a fairly large vocabulary. Some sounds are:

The "Wheek". A loud noise that sounds about the same as its written form. An expression of general excitement, it can mean "feed me", "pay attention",or possibly "I'm hurt". It is sometimes used to find other guinea pigs if they are in a run. If a guinea is lost, it will "wheek" for assistance.
"Rumbling". This sound is related to Guinea pig dominance or in response to an unfamiliar sound. It can also come as a response to comfort or content. Whilst courting, a male usually purrs deeply, while swaying from side to side, nearly lifting the rear feet.
"Chattering". This sound is made by rapidly gnashing the teeth together—it's a warning to others to keep away. Guinea pigs tend to raise their heads when making this sound so as to look more dangerous (it doesn't work—they still look like Guinea pigs).
"Tribbling". This rather pleasant sound is made when the Guinea pig is enjoying itself, usually when being petted or brushed. They may also make this sound when grooming.

The gestation lasts from 63-72 days, which is quite long for such a small animal. As a consequence pups are already well developed (including fur, teeth, claws and full eyesight) when they are born. The young are mobile from birth, and depending on the environment, will usually venture outdoors within a week. Pups begin eating solid food after a couple of days, though continue to suckle also. Males may demonstrate courtship behavior (following young females and making a "warbling" sound) in the second or third week after birth. Females can breed when only three weeks old. Litters vary from two or three young to as many as eight or more. In smaller litters, difficulties may occur during labour, due to oversized pups. Up to half the young may be lost in larger litters, as the mother cannot attend to the newborns fast enough. If a large number survive, it is likely that the weakest (or runts of the litter), will be insufficiently nursed, resulting occasionally in the death of one or two pups.