Interrelation between the CN-sensitive cytochrome path and the CN-resistant, benzohydroxamic acid (BHM)-sensitive, or n-propylgallate (nPG)-sensitive alternative path in seed respiration during germination was examined using the nondormant upper and lower seeds of Xanthium pensylvanicum Wallr. The operation of both paths was required not only for normal germination of the lower seed but also for KCN- or NaN(3)-induced germination of both. From the sensitivity to BHM of the germination response, it became obvious that the alternative path exerts its physiological activity as soon as it develops during the early period of water imbibition. Pretreatments with KCN and NaN(3) for promoting germination, strikingly decreased only the engagement of the cytochrome path in the subsequent respiration without affecting that of the alternative path. Nevertheless, no germination occurred without the operation of the cytochrome path. This suggested that excess operation of the cytochrome path is detrimental to germination, being maximal following the BHM-sensitive phase.The alternative and cytochrome paths operated in a proportion of 1 to 5 in a period just before the lower seeds started to germinate. However, there was little difference between the upper seeds of relatively low germination potential and the lower seeds of relatively high potential with respect to both the developmental pattern of the alternative path and the balance of the alternative and cytochrome paths. The higher germination potential of the lower seeds may be related to their high capacities for the alternative path.