The possibility that topically-applied minoxidil might affect the growth cycle of hair follicles was studied in inbred Herston white mice and HRA/Skh1 hairless mice. In the normal follicular cycle, the anagen or growth phase can be followed by autoradiographic demonstration of [3H]thymidine uptake in proliferating matrical cells, and the catagen or regression phase can be recognised, using light microscopy, by the presence of greatly increased death of matrical cells by apoptosis. Using these two markers, the effects of topically-applied minoxidil on follicular kinetics were studied, during neonatal hair growth and the spontaneous wave of hair loss that occurs 16 to 17 days after birth. Minoxidil at strengths of either 1% or 3%, applied daily to the dorsal skin of newborn mice from birth until 25 days of age, was found to have no recognisable effect. Despite this negative result, however, the study does show the potential for the use of apoptosis as a marker for catagen in research in dermatopathology.