Management of prematures in the NICU has reduced neonatal mortality and morbidity; however, deficits in cognitive and sensory functioning persist. The environment of the NICU may be responsible for newly recognized iatrogenic problems and may not be conductive for optimal development. We studied the quantity, quality, organization, and diurnal rhythmicity of physical and social stimulation in the NICU. Observations were conducted every half hour over 3 days. Physical recordings included light and sound levels, sound spectra, and occurrence of nonspeech, speech, and radio sounds. This information was collected both in the units and in incubators. Social data included the frequency of nursing care, feeding, social touching, rocking, and talking when in contact with an infant. There were 405 recordings for each physical variable and 1551 infant observations. The same illumination was always present. Infants were exposed incessantly to nonspeech sounds. Sound levels at times were excessively high. Incubators did not shield infants from stimulation. 19% of the observations included social contact of which 17% involved handling. Infants in the NICU suffered not from a lack of visual auditory, and tactile stimulation, but from a low frequency of sensory coordinated experiences. There was no diurnal rhythmicity in the physical or social stimulation across days. The nature of environmental stimulation in NICU's may contribute to the deficits associated with prematurity.