Circannual changes in the duration of the immobility response of rats in the forced swim test.

Separate groups of male rats were tested in the forced swim test on the third Tuesday of each mo for 14 mo. A distinct pattern was observable in their immobility response such that animals were most immobile during the winter months (Dec-Mar) and least immobile during the summer months (June-Sept). Bicarbonate-carbon dioxide levels were significantly correlated with immobility times (r = 0.48), as were glucose (r = 0.34) and phosphorus (r = -0.33) levels, whereas corticosterone and testosterone levels were not significantly correlated with immobility. These results indicate that there is a circannual rhythm in the floating behavior of rats in the forced swim test.