Paracetamol (N-acetyl-p-aminophenol) given i.p. in doses of 3–20 mg./kg./24 hr. stimulated epiphysial cartilage growth in intact female rats. This stimulation was not a result of increased food intake. It may be mediated by the adrenal gland, since paracetamol stimulated cartilage growth in thyroidectomized but not in adrenalectomized rats. Of 12 paracetamol derivatives tested, most did not affect cartilage growth or were less active than paracetamol. A structure-activity relationship could be established. Our results suggest that the mechanism of the stimulatory effect of paracetamol on growth may be twofold: the drug probably stimulates somatotrophin (STH) production and/or potentiates STH action on growth.