Relative nostril efficiency (nasal cycle) is related to hemispheric EEG differences and performance on cognitive tasks. We investigated how unilateral forced nostril breathing influences spatial and verbal performance. Right-handed males and females performed both tasks under either left-nostril, right-nostril, or free-breathing conditions. Unilateral breathing affects performance differently in males and females. It influences male performance ipsilaterally on both tasks: Their spatial performance is better during right-nostril breathing, and their verbal performance is better during left-nostril breathing. Unilateral breathing influences female performance contralaterally, but only on the spatial task: Their spatial performance is better during left-nostril breathing. These differences within and between sexes may exist because unilateral nostril breathing differentially activates the two hemispheres and thereby facilitates performance, or because attempts of the brain to control the nasal cycle unilaterally interfere with performance. D 19x9 Academic Press, Inc.There are various physiological, behavioral, and cognitive rhythms with a period of approximately 90-100 min, varying from about 80 min to about 120 min. Some cognitive variables that show this kind of ultradian rhythm are visual detection errors, fantasy production, response latency, and spatial and verbal performance (see and Rossi, 1986, for reviews). , for example, reported an approximately 90-min daydreaming cycle. found that performance on both spatial and verbal tasks followed approximately 96-min cycles and that when performance on one kind of task was best, performance on the other kind of task was worst. Their findings suggest that there are "oscillations in the relative activation or efficiency of the two cerebral hemispheres, which are specialized for the