A correlation was established between the time of the Achilles reflex and the biochemical characteristics of thyroid function (total thyroxin and triiodothyronine levels, thyroxin-binding capacity of the blood serum proteins, the basal TTH level) in patients with grave and moderately expressed hypothyroidism. This correlation was retained during the substitution therapy: however, the reflex time recovery was retarded as compared to the degree of manifestation of the clinical symptoms and normalization of the biochemical parameters. The time of the Achilles jerk may serve as an additional criterion in evaluating the hypothyrosis severity and the effect of the treatment.