Immobility time in 5d-FSSM. A, B, Six-week-old C57BL/6 mice were forced to swim on 5 successive days (D) for 10 min at ZT1 (A; n = 6) or ZT 14 (B; n = 6) following the advice of Sun et al. (2011). Animals were then tested on days 12 and 26. The immobility time was measured over the 10 min of swimming, but only the first 4 min were analyzed. A, Repeated one-way ANOVA revealed an effect of time: *p < 0.01 vs day 1; post hoc Tukey–Kramer test showed a difference between D1 and D4. B, On day 40, mice (n = 6) received injections of 8-OHDPAT (1 mg/kg, i.p.), a prototypical 5-HT1A receptor agonist, 30 min before the session. Another group (striped bar, n = 6) did not receive 8-OHDPAT injection or undergo swim stress on day 40 but was forced to swim again on day 61. Repeated one-way ANOVA showed a main effect of time. **p < 0001, *p < 0.05 vs day 1, #p < 0.05 vs D61. Immobility time was significantly shorter on day 1 than on days 3–33. The injection of 8–OHDPAT significantly decreased the immobility time. Results are expressed as the mean values ± SEM.