Intracellular calcium responses are a characteristic of glial activation upon neuronal activity. In acutely isolated preparations of the guinea pig retina, Müller glial cells displayed cytosolic calcium rises in response to repetitive light stimulation. The calcium rises consisted of two components, a slowly developing immediate response that occurred simultaneously over the whole length of all Müller cell fibers and a delayed fast response that originated in the ganglion cell layer and spread as a wave through the bodies of some Müller cells toward the outer processes in the photoreceptor layer. The slow calcium response was evoked by photoreceptor-to-glia signaling, resulting in a glutamate transporter- and zinc-mediated alteration in the membrane potential and an influx of calcium from the extracellular space. The fast calcium response was evoked by a release of calcium from intracellular stores, probably after activation of purinergic receptors. The data suggest that light stimulation of the retina causes glial activation by alterations in both the membrane potential and receptor-mediated mechanisms. The former may be implicated in glial support of the neuronal signal transfer from photoreceptors to ganglion cells (glial forward signaling), whereas the latter may constitute a glial feedback signaling from ganglion cells to photoreceptors.