Fura-2 and indo-1 fluorescence measurements were used to examine intracellular Ca++ concentration ([Ca++]i) and its modulation by voltage-activated influx in murine cortical astrocytes in primary cell culture. Extracellular K+ was increased from 5 to 50 mM to depolarize cells to determine if Ca++ influx through voltage activated Ca++ channels could alter [Ca++]i. In confluent 4 to 6 weeks in vitro astrocyte cultures 50 mM K+ increased [Ca++]i 3-4-fold (from 150 nM up to 550 nM); this increase was blocked by nifedipine and enhanced by BayK 8644 indicating that influx was through L-type channels. However, in 1 to 2 weeks in vitro astrocyte cultures, high K+ reduced [Ca++]i. L-type channels were apparently present in these cells because high K+ in combination with BayK 8644 increased [Ca++]i. Following pretreatment of 1 to 2 weeks in vitro astrocytes with dibutyryl cAMP (dbcAMP) high K+ increased [Ca++]i in the absence of BayK 8644 indicating enhanced activity of Ca++ channels in agreement with previous voltage-clamp studies. Ca++ influx through voltage-activated channels in cultured cortical astrocytes can substantially increase [Ca++]i and these channels can be dynamically modulated by dihydropyridines. Immature astrocytes may express 'silent' or inactive Ca++ channels or have a much lower number of channels.