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j. Soc. Cosmet. Chem., 38, 77-82 (March/April 1987) Measurement of the rate of hair growth using a fluorescent tracer technique JONATHAN R. MATIAS, RHODA ALANI, and NORMAN ORENTREICH, Biomedical Research Station, Orentreich Foundation for the Advancement of Science, RD 2, Box 3 70A, Cold Spring, NY 10516 (J.R.M., R.A., N.O.), Berg Institute, New York University Medical Center, 550 First Avenue, New York, NY 10016 (J.R.M.), and Department of Dermatology, New York University School of iMedicine, 550 First Avenue, New York, NY 10016 (N.O.). Received September 25, 1986. Synopsis The rate of hair growth was measured in the guinea pig by determining the distance between the fluores- cent bands produced by subcutaneous injections of sodium fluorescein. The minimal dose required to produce a visible fluorescent band was determined to be 30 mg/kg for the guinea pig. Rate of hair growth may also be measured in localized areas of the skin using intradermal injections at dosages as low as 0.25 mg. This method is simple, inexpensive, accurate, and less time-consuming than radiotracer methods. INTRODUCTION The measurement of the rate of hair growth is important in the evaluation of hormonal, cosmetic and drug effects on hair growth. Over the years a number of methods have been developed in an attempt to measure this parameter accurately. Notable among these are the capillary method by Saitoh et al. (1), the photographic technique by Burgess and Edwards (2), or the 35-S radioautographic method by Harkhess and Bern (3). The major disadvantages of the first two methods are that: (a) it is difficult to measure the rate of growth of the same hairs over a period of time, and (b) they cannot be easily adapted for hair growth measurements in animals with dense hair coat. These problems may be avoided by the injection of radiolabeled cystine which is incorporated into the hair shaft during the growth phase or anagen cycle. This method has been used successfully to measure rate of growth in sheep (3), guinea pigs (4), rats, and human beings (5). However, this technique is less convenient for the routine evaluation of drugs which might influence the rate of hair growth because of: (a) the prohibitive cost of isotope procurement and disposal, and (b) the long waiting time before the radioau- tographs may be developed. Fluorescein binds readily to the epidermal cells of the skin. Since this chemical may be 77