Previous pharmacological, biochemical and molecular evidence prove that μ-subtype opiate receptors and opiate alkaloids, i.e. morphine, are present in the ganglionic nervous system of the mollusk Mytilus edulis (bivalve). We now present molecular evidence on the effect of rapid temperature changes on μ opiate receptor expression and morphine levels. Using primers, a labeled Taq-Man probe derived from the human neuronal μ1 opiate receptor, and real-time RT-PCR to measure the expression of μ transcripts from Mytilus pedal ganglia, we observe, in animals placed in cold water from room temperature, an enhanced morphine and morphine 6 glucuronide level in addition to a decrease in μ opiate receptor gene expression. This study provides further evidence that μ-type opiate receptors and morphine are expressed in mollusk ganglia and appear to be involved in physiological processes responding to thermal stress.