The locus coeruleus (LC) has been suggested as a CO₂ chemoreceptor site in mammals. This nucleus is a mesencephalic structure of the amphibian brain and is probably homologous to the LC in mammals. There are no data available for the role of LC in the central chemoreception of amphibians. Thus, the present study was designed to investigate whether LC of toads (Bufo schneideri) is a CO₂/H⁺ chemoreceptor site. Fos immunoreactivity was used to verify if the nucleus is activated by hypercarbia (5% CO₂ in air). In addition, we assessed the role of noradrenergic LC neurons on respiratory and cardiovascular responses to hypercarbia by using 6-OHDA lesion. To further explore the role of LC in central chemosensitivity, we examined the effects of microinjection of solutions with different pH values (7.2, 7.4, 7.6, 7.8 and 8.0) into the nucleus. Our main findings were that: (i) a marked increase in c-fos positive cells in the LC was induced after 3 h of breathing a hypercarbic gas mixture; (ii) chemical lesions in the LC attenuated the increase of the ventilatory response to hypercarbia, but did not affect ventilation under resting conditions; and (iii) microinjection with acid solutions (pH= 7.2; 7.4 and 7.6) into the LC elicited an increased ventilation, indicating that LC of toads participates in the central chemoreception.