An experimental study was performed to investigate the influence of hypothyroidism on wound healing. A state of severe hypothyroidism was induced initially by performing a total thyroidectomy on rat models; subsequently wounds were made by making long midline abdominal incisions and then suturing them. The parameters used to evaluate the process of wound healing of these incisions were the assay of type - I collagen, type-III collagen (procollagen peptide PIPC and P III P, each being the precursor of collagen), type-IV collagen, and hydroxyproline. The assays were repeated at specific times and compared to assays of similar parameters taken from a control group. In the state of hypothyroidism, a decrease was observed in type-IV collagen and hydroxyproline during the proliferative phase of wound healing. This indicated that the state of hypothyroidism constitutes an important factor in delaying wound healing. (J Nippon Med Sch 1999; 66: 176-180)