Large calyciform synapses in the rat reticular thalamic nucleus are characterized by the presence of γ-aminobutyric acid. Presynaptic terminals are also loaded with calcium-binding proteins such as parvalbumin, calbindin, calretinin and calcineurin. The number of calyciform terminals containing γ-aminobutyric acid and parvalbumin is 2005 in young adult rats; calbindin is present in 1500, calretinin in 850 and calcineurin in 560 calyciform terminals. Developmental studies revealed that γ-aminobutyric acid and calcium-binding proteins are virtually absent from calyciform terminals at birth but their occurrence increased considerably during postnatal life, suggesting increasing regulation of presynaptic calcium signaling during postnatal life. It is concluded that synaptic activity of large calyciform γ-aminobutyric acid-containing synapses of the reticular thalamic nucleus is mediated, regulated or accompanied by calcium ions.