Heart rate in vertebrates is controlled by activity in the autonomic nervous system. In spontaneously active or experimentally prepared animals, inhibitory parasympathetic control is predominant and is responsible for instantaneous changes in heart rate, such as occur at the first air breath following a period of apnoea in discontinuous breathers like inactive reptiles or species that surface to air breathe after a period of submersion. Parasympathetic control, exerted via fast-conducting, myelinated efferent fibres in the vagus nerve, is also responsible for beat-to-beat changes in heart rate such as the high frequency components observed in spectral analysis of heart rate variability. These include respiratory modulation of the heartbeat that can generate cardiorespiratory synchrony in fish and respiratory sinus arrhythmia in mammals. Both may increase the effectiveness of respiratory gas exchange. Although the central interactions generating respiratory modulation of the heartbeat seem to be highly conserved through vertebrate phylogeny, they are different in kind and location, and in most species are as yet little understood. The heart in vertebrate embryos possesses both muscarinic cholinergic and beta-adrenergic receptors very early in development. Adrenergic control by circulating catecholamines seems important throughout development. However, innervation of the cardiac receptors is delayed and first evidence of a functional cholinergic tonus on the heart, exerted via the vagus nerve, is often seen shortly before or immediately after hatching or birth, suggesting that it may be coordinated with the onset of central respiratory rhythmicity and subsequent breathing.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)National Science FoundationUniv Estadual Paulista, Dept Zool, Inst Biociencias, BR-13506900 Sao Paulo, BrazilUniv Birmingham, Sch Biosci, Birmingham B15 2TT, W Midlands, EnglandUniv Fed Sao Paulo, Dept Ciencias Biol, BR-04021001 Sao Paulo, BrazilNatl Inst Sci & Technol Comparat Physiol, Sao Carlos, SP, BrazilAarhus Univ, Dept Biosci, DK-8000 Aarhus, DenmarkUniv N Texas, Dept Biol Sci, Dev Integrat Biol Cluster, Denton, TX 76203 USAUniv Estadual Paulista, Dept Zool, Inst Biociencias, BR-13506900 Sao Paulo, BrazilCNPq: INCT 573921/2008-3FAPESP: 08/57712-4FAPESP: 10/51995-4FAPESP: 08/00107-1FAPESP: 12/06938-8FAPESP: 12/16537-0National Science FoundationIOS-0845741