In this research, the changes of anxiety and blood progesterone levels during the oestrus cycle were studied in rats genetically selected for high (KHA) and low (KLA) acquisition of active avoidance. Anxiety levels were measured by the time spent in open arms of the elevated plus-maze. Progesterone levels were determined by radioimmunoassay. KLA rats exhibited no significant changes in anxiety levels during the oestrus cycle. KHA rats showed a significant variation of anxiety during the oestrus cycle with a high level in the diestrus phase and a low level in proestrus. Moreover, anxiety in diestrus in KHA rats was higher than in KLA rats. Additionally, increased progesterone levels were observed in KLA rats in comparison with the KHA strain, during both diestrus and proestrus. Anxiety levels corresponded to plasma progesterone during the oestrus cycle in both rat strains.