The cuticle is the prime barrier to penetration of foliar applied plant growth regulators, which must penetrate and be transported to a reaction site before a response can be induced. Urea has enhanced performance of Fe and Zn foliar sprays and a mixture of urea and ammonium nitrate (WAN) the performance of some herbicides. The mechanism of this enhancement is not clear. We find that urea and UAN increased 14C-NAA transport across enzymatically isolated tomato fruit cuticular membranes (CM) from simulated spray droplets using a finitedose diffusion system. The initial rate and total amount of NAA penetrated was significantly increased relative to NAA alone, the enhancement being greater for UAN than urea (total amount 101% vs. 78% at 120 hours) and for the NAA anion (pH 5.2, pKa 4.2) than for the nondissociated (pH 3.2) moiety. When evaluating the concentration effect of urea and NH4NO3 individually, the greatest enhancement with urea was at 62 mm and with NH4NO3 at 8 mm. Generally the effect of urea was significantly less than NH4NO3 (+24% vs. 296%). NAA penetration was greater with NH4NO3 than with KNO3 or Ca(NO3)2 or when the nitrate anion was replaced with sulfate or phosphate. Transcuticular penetration of NAA was enhanced greatly (190% in 120 hours) on removal of cuticular waxes; however, penetration was further increased (252% in 120 hours) by adding 8 mm NH4NO3. Methylamine hydrochloride (CH3NH2.HC1, 8 mm) also increased NAA diffusion, the initial slopes (>8 hours) were 23, 14, and 2 pmols·h–1 for methylamine, ammonium nitrate, and NAA alone, respectively, while the percent of applied that penetrated after 120 hours was 68.5, 67.6, and 21.4 for methylamine, ammonium nitrate, and NAA alone, respectively. The enhancement of NAA penetration by NH4NO3 equaled or exceeded that obtained with a group of surfactants of diverse chemistries. When the surfactant Triton X-100 was compared with NH4NO3, initial penetration was more rapid with ammonium nitrate (11.7 vs. 7.3 pmols·h–1) but percent penetrating after 120 hours was greater for Triton X-100 (80.5 vs. 66.8). The possible action of NH4NO3 on NAA uptake will be discussed.