Figure 2. Effect of naloxone on antinociceptive response to nitrous oxide. Three cohorts of rats (n = 6 per group) were given saline or naloxone, 0.2 or 1.0 mg kg sup -1, administered intraperitoneally 30 min before the latency to elicit a tail flick in response to a heat stimulus was measured. Subsequently, the animals were exposed to nitrous oxide (70% v/v) for 30 min and the tail-flick latency measurement was repeated. Data are expressed as mean +/- SEM. *= significantly different (P < 0.05) from baseline measurement.