The main test consists of a single ordered dose progression in which animals are dosed, one at a time, at a minimum of 48-hour intervals. The first animal receives a dose a step below the level of the best estimate of the LD50. If the animal survives, the dose for the next animal is increased by [a factor of] 3.2 times the original dose; if it dies, the dose for the next animal is decreased by a similar dose progression.
The preferred rodent species is the rat although other rodent species may be used. Normally female rats are used. This is because literature surveys of conventional LD50 tests show that usually there is little difference in sensitivity between sexes, but in those cases where differences are observed, females are generally slightly more sensitive.
The LD50 is calculated using the method of maximum likelihood.
Further information: http://www.oecd.org/dataoecd/17/51/1948378.pdf