The opening of intracellular potassium channels his been suggested as a mechanism regulating hair growth. Enhancing the flu of potassium ions is a mechanism shared by several structurally diverse antihypertensive agents including minoxidil sulfate (the active metabolite of minoxidil), pinacidil, P 1075 (a potent pinacidil analog) RP-49 356, diazoxide, cromakalim, and nicorandil. Of these drugs minoxidil, pinacidil, and diazoxide have been reported to elicit hypertrichosis in humans, This potassium channel hypertrichosis was examined by testing these drugs for effects on hair growth o h in vitro and in vivo. For the in vitro studies, mouse vibrissae follicles were cultured for 3 d with drug and the effect on hair growth were measured by metabolic labeling. All drugs, except diazoxide, ecchanged cysteine incorporation o the hair shafts of the cultured vibrissae. Diazoxide was in vivo by measuring hair growth effects in balding strumptail macaque monkeys. The drugs were administered topically to defined sites on balding scalps once per day for 4- 5 months and the amount of hair grown was determined by monthly measurements of shaved hair weight. Three of the drugs produced significant increases in hair weight whereas, the RP – 49, 356 had no effect. These studies provide correlative evidence that the opening of potassium channels is an important regulatory mechanism for hair growth. This provides the impetus for further studies on this potentially important mechanism affecting hair biology.