INTRODUCTION: The formalin test is the most accepted chemical test for 
evaluation of nociception. It requires the injection of an adequate amount of 
formalin into the surface of the hindpaw. Formalin test consists of phase 1 (0-7 
min) and phase 2 (15-60) in which the animal shows painful behaviors. These 
phases are separated with a quiet phase named interphase, in which the 
nociceptive responses are decreased or completely disappeared.
METHODS: The goal of the current study was to evaluate the effects of swim 
stress at different heights of water on different phases of the formalin test in 
male rats.
RESULTS: Swim stress decreased nociceptive behaviors in first phase and 
prolonged interphase or delayed the start of second phase in a water height 
dependent manner. Swim stress in 25 and 50cm completely abolished the 
nociceptive behaviors in phase 1.
DISCUSSION: The present results showed different pain modulation during 
different phases of the formalin test and elucidated the impact of swim stress 
on duration of interphase. Interphase considered as an inactive period, but a 
recent research has shown that active inhibitory mechanisms are involved in the 
modulation of pain during this period. Therefore, swim stress may be considered 
as a useful tool for study of the basic inhibitory mechanisms underlying 
attenuation of nociceptive behaviors between phase 1 and 2 of the formalin test.