We studied the effects of adjuvant arthritis (AA) on the endocrine circadian rhythms of plasma prolactin (PRL), growth hormone (GH), insulin-like growth factor-1 (IGF-1), luteinizing hormone (LH), testosterone, and melatonin and of pituitary PRL and GH mRNA in male Long Evans rats. Groups of control and AA rats (studied 23 days after AA induction) that were housed under a 12/12 h light/dark cycle (light on at 06:00 h) were killed at 4 h intervals starting at 14:00 h. Cosinor analysis revealed a significant 12 h rhythm in PRL and PRL mRNA (p < 0.001) in controls with peaks at 14:00 h and 02:00 h, respectively. The peak at 02:00 h was abolished in the AA group resulting in a significant 24 h rhythm in parallel with that of PRL (p < 0.05) and PRL mRNA (p < 0.0001). Growth hormone showed no rhythm, but a significant rhythm of GH mRNA was present in both groups (p < 0.0001). Insulin-like growth factor-1 showed a 24 h rhythm in control but not in AA rats. The mean values of GH, GH mRNA, and IGF-1 were significantly reduced in AA. Luteinizing hormone displayed a significant 24 h rhythm (p < 0.01) peaking in the dark period in the control but not AA group. Testosterone showed in phase temporal changes of LH levels with AA abolishing the 02:00 h peak. Melatonin exhibited a significant 24 h rhythm in control (p < 0.001) and AA (p < 0.01) rats with maximum levels during the dark phase; the mesor value was higher in the AA males. These results demonstrate that AA interferes with the rhythms of all the studied hormones except the non-24 h (arrhythmic) GH secretion pattern and the rhythm in melatonin. The persistence of a distinct melatonin rhythm in AA suggests the observed disturbances of hormonal rhythms in this condition do not occur at the level of the pineal gland.