The duration of pain relief after 1st dose was statistically significant and was 339.13 + / - 79.57 minutes in group B and 709.75 + / - 72.12 minutes in group M which was gradually increased on repeated dosing in group B while it was almost same in Group M. Number of doses required in 24 hours was significantly higher ( p < 0.05 ) in butorphanol group than morphine group .