We have studied the transdermal flux of prostaglandin $E_1$ $(PGE_1)$ from a hydrogel patch through hairless mouse skin, to test the possibility of developing a transdermal delivery system. Karaya gum patch containing $PGE_1$ was prepared by casting method. $PGE_1$ was stable in the patch for 10 weeks. The effect of current application, enhancer (propylene glycol monolaurate : PGML), adhesive and patch thickness on the flux was studied using side-by-side diffusion cell. Passive flux of $PGE_1$ was negligible. Cathodal delivery increased the flux about 20 fold. As the concentrations of PGML increased, flux increased. When 5% PGML was used as the enhancer, maximum flux by cathodal iontophoresis was $55;{mu}g/cm^2;hr$. It increased about 2 folds to $100;{mu}g/cm^2;hr$, when the amount of PGML used was 9%. Large increase in flux and the decrease in time to reach maximum flux were observed when the skin was pretreated with neat PGML (maximum flux obtained was about $200;{mu}g/cm^2;hr$). Use of adhesive decreased the flux significantly. To the contrary of our expectation, increase in current density decreased the flux. These flux data together with the stability data indicate that, though the onset of sufficient delivery occur after 1-2 hours of application, therapeutic amount of $PGE_1$ can be delivered through skin using iontophoresis and penetration enhancer.