Breathing is controlled by inspiratory pre-Bötzinger complex (preBötC) networks that remain active in transversal brainstem slices from perinatal rodents. In 600 microm thick preBötC slices, inspiratory-related bursting in physiological (3mM) [K(+)] is depressed by <1mM elevation of superfusate [Ca2+]. Here, we studied underlying cellular mechanisms in whole-cell-recorded neurons of 400 microm thin newborn rat slices with the <200 microm thin preBötC in the middle ("m-preBötC[400]" slices). Extracellular activity in the ventrolateral slice area in 3mM K+ and a most common physiological Ca2+ range (1-1.2mM) stopped spontaneously within 2h ("in vitro apnea"). Contrary, rhythm was stable for >3h at 6-8 bursts/min in 7 mM K+ and 1.2mM Ca2+ solution. In non-pacemaker preBötC inspiratory cells and neighboring inspiratory or tonically active neurons, block or frequency depression by >90% of rhythm in the latter solution by 2-3mM Ca2+ changed neither resting potential nor input resistance. High Ca2+ silenced inspiratory neurons and depressed tonic discharge of non-respiratory neurons. However, in both cell types current injection evoked normal action potentials with unchanged threshold potential. The findings show that m-preBötC[400] slices represent a good compromise between long term viability of rhythmogenic preBötC neurons and minimal modulation of these cells by adjacent tissue, but need to be studied in elevated K+. The lack of postsynaptic K+ channel-mediated hyperpolarization suggests that saturation of surface charges, presynaptic block of transmission and/or inhibition of postsynaptic burst-promoting conductances such as Ca2+ activated non-selective cation channels are involved in inspiratory depression by high Ca2+.