Optimal electrolytes: The standard medical ranges for optimal salts, acids, and bases (which together we call "electrolytes") are given below.

These concentrations are in millimolar (millimoles per liter)...note the big differences between what your cells like inside and outside...to keep this difference requires a lot of energy!!

extra cellular (mmol/L)	Sweat (mmol/L)	Intracellular (mmol/L)
Sodium Na+ 137-144	20-80	10
Potassium K+ 3.5-4.9	4.0-8.0	148
Calcium Ca++ 4.4-5.2	3.0-4.0	0-2.0
Magnesium Mg++ 1.5-2.1	1.0-4.0	30-40
Chloride Cl- 100-108	30-70	2

You asked about "during endurance exercise"...but that is a much more difficult question. Here's why: The body functions best when these concentrations are maintained in the blood. During long periods of intense exercise, it is possible to lose more water than normal, which increases concentration of various electrolytes (because the amount of solutes hasn't changed as much as the reduction in solvent/water). This leads to "dehydration" where your body begins to suffer chemical imbalances because the concentrations of electrolytes are too high and there is insufficient fluid to flush the body systems. Unfortunately, the optimal addition depends on the person and the exercise.

Most sport drinks are designed to either rapidly replace just electrolytes, or to slowly replace electrolytes and provide sugars over the long haul. The way this is accomplished is by making sports drinks either hyper- or hypo-tonic...hyper being "over" and hypo being "under"...the normal concentration of your blood. If the drink is hypotonic, it has less sugars and salts than your blood, causing water to rush into the bloodstream and rapidly re-hydrate. If the drink is hypertonic, it is loaded with sugar and therefore will take a long time to be absorbed...in addition this sugar can be metabolized. Generally these are the drinks you would use for an endurance sport so that you maintain hydration over a long period and provide continuous energy.

You can compare the typical concentrations of things in two types of electrolyte drinks...note that these concentrations are in milligrams per liter not millimoles per liter, and the endurance drink will also have a lot of sugar/carbohydrates, which are not considered electrolytes but do affect both metabolism and the rate of absorption into the bloodstream:

Electrolyte	Sweat Loss mg/L	Standard Sport Drink mg/L	Endurance Specific Sport Drink mg/L
Sodium	900-2600	200-450	800-1110
Potassium	100-200	80-125	390-650
Magnesium	60-260	0	10-615
Chloride	900-1900	0	390-1550
Calcium	50-100	0	250-500

The optimal serum sodium level is 135-145mmol/L. At about 125-134 mmol/L people start to develop symptoms of hyponatremia (low sodium levels,) which causes nausea, headache, dizziness.