The pineal gland hormone was shown to be involved in the regulation of repair and connective tissue accumulation in a wound. The hypothesis that melatonin injection could reverse the pinealectomy-induced elevation of the collagen content in scar tissue was verified. The effect of various melatonin doses (3, 30 and 100 micrograms/100 g body wt) on soluble, insoluble and total collagen content in the granulation tissue of a wound was investigated. The collagen level was estimated in pinealectomized rats, as well as in pinealectomized animals treated with melatonin. Ivalon sponges inserted subcutaneously were applied as a wound model. After 4 weeks the collagen content was measured as a hydroxyproline concentration in the dry tissue of the wound. The results showed that melatonin at the dose of 30 micrograms/100 g body wt significantly decreased both total and insoluble collagen content in the wounded tissue (p < 0.05), but other doses were ineffective. Pinealectomy increased the total (p = 0.05) and insoluble collagen level (p < 0.05) in the granulation tissue. Melatonin suppressed pinealectomy-induced elevation of the total and insoluble collagen content in the wounds (p < 0.05). No influence of the pineal gland on the soluble collagen content was observed. The results showed that melatonin was involved in the inhibitory control of the collagen content.