Metaphase nucleolar organizer region (NOR) activity was studied in mouse lines selectively bred for high analgesia (HA) and for low analgesia (LA) induced by 3-min swimming in 20 degrees C water. Apart from pain-related traits, HA mice also manifest, as compared to the LA line, higher emotionality in certain behavioral tests and are less capable of coping with the hypothermic challenge of swimming in cold water, in addition to being more susceptible to the mutagenic effect of whole-body gamma-radiation and mitomycin C injection. We here compared NOR activity in HA and LA mice. A statistically significant difference (p < or =.01) was detected in mean silver-stained nucleolar number/cell between the HA and LA lines, being lower for HA mice. We propose that the breeding strategy, along with the differentiation of stress-related phenomena, has altered the activity of genes coding rRNA and that this activity is important in controlling DNA repair in each line. It is concluded that quantitative studies of nucleolar changes may be useful in evaluating the biological reactivity to stress stimuli.