The G- and U-specific reagent, carbodiimide was used to probe the solution structure of aminoacylated yeast phenylalanine tRNA. Both quantitative and qualitative changes in modification were observed when the modification patterns of tRNA-CCA(3'OH), tRNA-CCA(3'NH 2 ) and phe-tRNA-CCA(3'NH 2 ) were compared. Five nucleotides were modified in all cases, D16 and G20 in the D-loop, U33 and Gm34 in the anticodon loop and U47, in the region of the extra arm. Small changes occurred in the D-loop with incorporation of the adenosine analogue manifest as new, low levels of modification of G22 (D-stem) and a loss of sensitivity to Mg +2 in modification of D16. Aminoacylation resulted in new modification of G19, modification of a residue in the TψCG sequence, and a 2.5-fold increase in modification of G22. Taken together the results show that sminoacylation causes increased exposure of bases in the apex region of the L-shaped molecule where the D- and ψ-loops are joined. The effects observed could occur an a consequence of stable or dynamic changes in conformation.