The expression of GFAP and bFGF after retinal photocoagulation injury in the rat was assessed by immunocytochemistry and reverse transcriptase PCR. Beta-actin mRNA expression was unaltered after injury and was therefore a good control for the quality of the RNA samples and the PCR reaction. GFAP mRNA increased from undetectable levels in normal retina to relatively high levels at 24 and 48 hr after injury, returning to barely detectable levels at 3 and 7 days. Muller cell GFAP immunoreactivity was elevated by 24 hr, stronger by 48 hr and persisted for 30-45 days. Thus, the expression of GFAP immunoreactivity after photocoagulation was due to new protein synthesis but the mRNA, and therefore the stimulus, was only present for a few days. This indicates that the GFAP protein in Muller cells has a long lifetime similar to that of astrocytes despite different gene control elements. bFGF is a possible stimulus for Muller GFAP expression because Muller cells have bFGF receptors.bFGF mRNA was detectable in normal, 24 and 48 hr retinas but decreased to undetectable levels (even after 35 cycles of PCR) at three days after injury and had only partly recovered by 7 days. Immunocytochemistry demonstrated a rapid change in localization of bFGF at the lesion sites early after lesion. At 2-3 days bFGF in blood vessels was markedly increased while at 7 days there was an increase around the photoreceptors flanking each lesion. These shifts in bFGF localization were too late to be a stimulus for the widespread upregulation of GFAP expression by Muller cells. The reduction in bFGF mRNA at three days was unexpected as studies of brain injury generally show a longer lasting elevation of bFGF expression. Therefore it is likely that bFGF expression is controlled by different mechanisms in the retina compared to the brain. However, a reduction in bFGF synthesis after photocoagulation is consistent with the anti-angiogenic effect of laser photocoagulation in diabetic retinopathy (C) 1997 Academic Press Limited.