The solution structure of porcine pancreatic phospholipase A2 (124 residues, 14 kDa) has been studied by two-dimensional homonuclear 1H and two- and three-dimensional heteronuclear 15N-1H nuclear magnetic resonance spectroscopy. Backbone assignments were made for 117 of the 124 amino acids. Short-range nuclear Overhauser effect (NOE) data show three alpha-helices from residues 1-13, 40-58, and 90-109, an antiparallel beta-sheet for residues 74-85, and a small antiparallel beta-sheet between residues 25-26 and 115-116. A 15N-1H heteronuclear multiple-quantum correlation experiment was used to monitor amide proton exchange over a period of 22 h. In total, 61 amide protons showed slow or intermediate exchange, 46 of which are located in the three large helices. Helix 90-109 was found to be considerably more stable than the other helices. For the beta-sheets, four hydrogen bonds could be identified. The secondary structure of porcine PLA in solution, as deduced from NMR, is basically the same as the structure of porcine PLA in the crystalline state. Differences were found in the following regions, however. Residues 1-6 in the first alpha-helix are less structured in solution than in the crystal structure. Whereas in the crystal structure residues 24-29 are involved both in a beta-sheet with residues 115-117 and in a hairpin turn, the expected hydrogen bonds between residues 24-117 and 25-29 do not show slow exchange behavior. This and the absence of several expected NOEs imply that this region has a less well defined structure in solution. Finally, the hydrogen bond between residues 78-81, which is part of a beta-sheet, does not show slow exchange behavior.