Morphological and morphometrical development of the neurons in the medullary reticular formation was observed in control, sudden infant death syndrome (SIDS) and prematurely born infants, using Golgi stains. With increasing gestational age, in controls dendrites became long, spines increased and the distribution of spines assumed a mature pattern. The number of spines reached a peak at 34 to 36 weeks gestation and after birth it decreased rapidly. Term and prematurely born SIDS infants showed a persistence of reticular dendritic spines. On the other hand, ventilator-dependent prematurely born infants had decreased numbers of dendritic spines with thin dendrites and long, thin spines. Persistence of reticular dendritic spines in SIDS infants may suggest incomplete development to the mature, higher level of respiratory control. Spine abnormalities in the ventilator-dependent prematurely born infants may indicate secondary neuronal changes in the reticular formation.