Activation of serotonin 1A (5-HT(1A)) receptors has been shown to have diverse effects on respiration. The purpose of this study was to determine changes in respiratory motor pattern of phrenic nerve activity and respiratory rhythm after systemic application of specific 5-HT(1A) receptor agonist 8-hydroxy-2-di-npropylamino-tetralin (8-OH-DPAT). We hypothesized that systemic application of specific 5-HT(1A) receptor agonist 8-OH-DPAT in spontaneously breathing anaesthetized rats will enhance phrenic motor output and phrenic respiratory rate. The study was performed in spontaneously breathing urethane anesthetized rats. Intravenous application of 8-OH-DPAT produced dose dependent increase in the amplitude of integrated phrenic nerve activity and disturbances in respiratory rhythm. Stimulating effect of 8-OH-DPAT on phrenic nerve activity was abolished by intravenous application of the selective 5-HT(1A) receptor antagonist WAY, N-(2-(4,2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2-pyridinyl-cyclohexane-carboxamide maleate (WAY-100635). These results show that stimulation of 5-HT(1A) receptors by intravenous application of 8-OH-DPAT enhances phrenic nerve activity in spontaneously breathing rats.