LD₅₀

LD 50 - "Median Lethal Dose" or dose which kills 50% of the animals tested.
Dose is expressed as:

mass of chemical (mg) / body weight of recipient (kg)

For example:
Human = 70 kg, Rat=0.25 kg (280-fold difference)

If each got 100 mg of aspirin:
Human dose = 0.7 mg/kg, rat dose = 400 mg/kg

Must state the specific parameters of the experiment:
Oral 14-day LD50 in female rat
(time, route of exposure, species, sex)

Usually performed on 10 rats per dose level (5 each sex), at least 3 doses.
Always a single dose!

Limitations: species differences, extrapolation, unrealistic exposures.