This paper distinguishes two models specifying the application of implicit knowledge. According to one model, originally suggested by Reber (1967), subjects either apply sufficient knowledge to always produce a correct response or else they guess randomly (High Threshold Theory; subjects only apply knowledge when there is sufficient knowledge to exceed a threshold ensuring a correct response); according to the other model, suggested by Dienes (1992), subjects respond with a certain probability towards each item, where the probability is determined by the match between the items structure and the induced constraints about the structure (Probability Matching Theory; subjects match their probability of responding against their personal probability that the item belongs to a certain category). One parameter versions of both models were specified and then tested against the data generated from three artificial grammar learning experiments. Neither theory could account for all features of the data, and extensions of the theories are suggested.