Organelles of multilamellate form, consisting of concentric layers of interconnected tubules, are present in the preterminal portions of neurosecretory fibres in the pericardial organs of Squilla mantis. Secretory material accumulates as spherical droplets within these organelles. A continuous series of forms intermediate between bodies resembling mitochondria and multilamellate organelles are demonstrated by electron micrographs. Both the mitochondria and the multilamellate organelles are continuous with an intricate system of microtubules in the preterminal portions of the neurosecretory fibres. These findings may be regarded as compatible with the hypothesis that secretory material assembles within mitochondria, or their derivatives, in the preterminal portions of neurosecretory fibres, and that the distal part of a neurosecretory system plays an active part in hormone transformation or assemblage.