The purpose was to examine sighs and spontaneous pauses in regard to the stability of resting breathing in the B6 strain, compared to the A/J strain. A 5-HT1A receptor agonist (buspirone) and a chromosomal substitution strain (Mal) were used to further alter breathing patterning. Ten-minute recordings of room air breathing were collected from unanaesthetized B6, A/J, and B6a1 mice. Despite no differences between strains in the magnitude and incidence of sighs, post-sigh apneas, the variation for duration of expiration (T-e) after sighs, and the number of spontaneous pauses were greater in the B6, while Shannon Entropy (nonlinear metrics) for T-e after sighs was lower in B6, compared to the other strains. Buspirone and chromosomal substitution eliminated post-sigh apneas and decreased spontaneous pauses. A greater irregularity and the lower complexity of post-sigh breathing in B6 are reversed by elements on A/J chromosome 1 and by increased 5-HT1A serotonergic tone. (C) 2008 Elsevier B.V. All rights reserved.