Subjected 137 male Holtzman albino rats to bilateral lesions of the medial forebrain bundle (MFB) or to surgical control procedures. Lesions produced equivalent decreases in paw-lick latency to heat and in jump and high-magnitude-response thresholds to footshock. Flinch and detection thresholds to footshock were also significantly decreased, though the lesion did not alter the magnitude or habituation of noise-elicited startle. Injection of 75 mg/kg serotonin into lesioned Ss returned the high-magnitude-response threshold to normal values but did not affect detection threshold. It is concluded that the increased sensitivity to painful stimuli resulted from the decrease in telencephalic content of serotonin following MFB lesions. Previous exposure of lesioned Ss to footshock resulted in a potentiation of the startle response to noise, further suggesting that the lesion had increased the aversiveness of footshock. (27 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved)