Figure 1. Effect of intravenously administered morphine alone on paw withdrawal response to noxious heat. Morphine (2.5 mg/kg) produced an increase in paw withdrawal latency, expressed as percent maximum possible effect (%MPE), which was stable for 60 min in the presence of repeated intrathecal injections of saline (top). Arrows indicate time of saline injection. Morphine antinociception was linearly related to the log of the dose (bottom). Data are presented as mean +/- SD.