Most in situ hybridization histochemistry studies of messenger RNA in human brain have been carried out on frozen tissue. Recently, autoclaving has been reported to enable routinely processsed material to be used for in situ localization of messenger RNA. We have investigated whether autoclaving also permits in situ hybridization histochemistry to be used quantitatively. To do this, we targeted synaptophysin messenger RNA with a 35S-labelled oligonucleotide probe in autoclaved, formalin-fixed, paraffin wax-embedded sections of the hippocampal formation of 11 schizophrenics and 11 controls. We compared the results with those seen on frozen sections from adjacent blocks, which had been used previously to demonstrate a loss of the messenger RNA in schizophrenia. Synaptophysin messenger RNA was readily detected in the autoclaved sections. The hybridization signal correlated strongly with that seen in the frozen sections. We found a similar pattern and magnitude of decreased synaptophysin messenger RNA in schizophrenia in the autoclaved sections as we had in the frozen sections, including the selective preservation of synaptophysin messenger RNA in CA1. The reduction of synaptophysin messenger RNA was replicated when six subjects with schizophrenia not included in the earlier study were considered separately. We conclude that autoclaving renders formalin-fixed, paraffin wax-embedded sections of human brain suitable for quantitative in situ hybridization histochemistry. This has considerable implications, given the wider availability, better morphology and easier handling of fixed than frozen human brain tissue. Using this material, we confirmed the finding of decreased synaptophysin messenger RNA in the hippocampal formation in schizophrenia, furthering the evidence for synaptic pathology in this region in the disorder.