The present study aimed to evaluate the acaricidal action of the chemically modified essential oil of Cymbopogon spp. and Corymbia citriodora on Rhipicephalus (Boophilus) microplus. Citronellal was converted into N-butylcitronellylamine and in N-prop-2-inylcitronellylamine, analogs of juvenoids, by reductive amination using butylamine (N1 to N3) and propargylamine (N4 to N7). In vitro assays included the adult immersion, and larval packet tests. Engorged females were weighed in groups of 10 and tested in three replicates for six concentrations. They were immersed in the modified oils or control solution and incubated. In the larval packet test, the same substances and concentrations were evaluated in three replicates. In the in vivo test, six pastured heifers naturally infested with R. (B.) microplus were used per treatment: negative control, positive control (amitraz, Triatox (R)), original oil of C citriodora at 1.5%, and modified oil containing 0.9% N-prop-2-inylcitronellylamine (N7). Ticks were counted in the right side of the body in 24 animals from day D-3 to D21. LC50 and LC90 were obtained by Probit analysis, while the in vivo results were log transformed and compared using the Tukey test. Among the nitrocellylamines tested in vitro, N6 was most effective on the engorged females (100% efficacy at 50 mg/mL) and N7 on the larvae (100% efficacy at 6.25 mg/mL). In the test with larvae, the original oil of C. citriodora was less effective than the counterpart modified oil (N7), proving that the chemical modification optimized its effect. In the in vivo test, no significant difference was observed between N7 and the negative control. The average numbers of ticks on the animals' right side were 32.8, 8.1,37.9 and 35.4 for the negative control, positive control, original oil and N7, respectively. The chemical modification improved the efficacy in vitro, but it was not observed in vivo, perhaps due to the low stability of the amines under field conditions. The evaluation carried out here has not been previously studied, so this concept expands the horizon for research into chemically modified substances for parasite control and shed light on the challenges to find effective formulations and application methods. (C) 2014 Elsevier B.V. All rights reserved.