These terms are best regarded as synonymous with @R05139@. However, other meanings that have been given to them should be mentioned, as it is necessary to be aware of them in order to avoid confusion. Sometimes the term rate-determining is used as a special case of rate-controlling, being assigned only to an initial slow step which is followed by rapid steps. Such a step imposes an upper limit on the rate, and has also been called rate-limiting. In view of the considerable danger of confusion when special meanings are applied to rate-determining and rate-limiting, it is recommended that they be regarded as synonymous, with the meaning explained under the entry @R05139@.