Phospholipase C (PLC) activity has been implicated in multiple opioid-induced sequelae. The relevance of PLC-linked pathways to opioid actions is isoform-specific. Chronic morphine augments PLCbeta1 signaling while diminishing that of PLCbeta3. This suggests that PLCbeta1 makes an important contribution to opioid tolerance formation (PNAS 100: 13686-1369, 2003). In the present study, PLCbeta1 knockout animals (-/-) were used to assess the relevance of PLCbeta1 to pain thresholds, morphine antinociception and analgesic tolerance formation. Response latencies to thermal nociceptive stimuli were markedly diminished in -/- animals relative to their wild-type (+/+) and heterozygous (+/-) counterparts; thermal nociceptive thresholds obtained in +/+ and +/- mice did not differ. This suggests that the contribution of PLCbeta1 to thermal pain thresholds requires a critical concentration of PLCbeta1 protein. PLCbeta1 genotype also influenced acute and chronic responsiveness to morphine. Analgesic dose responsiveness and the magnitude of analgesic tolerance formation to morphine were significantly attenuated in -/- vs. +/+ animals. Notably, in contrast to thermal nociceptive thresholds, acute and chronic morphine responsiveness differed significantly only between +/+ and -/- genotypes and not between -/- vs. +/- groups. These data suggest that whereas the contribution of PLCbeta1 to thermal nociceptive response thresholds requires a critical concentration of PLCbeta1 protein, its participation in morphine analgesic and tolerance-producing mechanisms is graded. Importantly, GTPgammaS binding studies revealed that there is no detectable diminution in functional opioid receptors in spinal tissue from -/- animals. This underscores the importance of PLCbeta1 to morphine sequelae that are initiated downstream from the opioid receptor.