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Strong spin fluctuations arising in itinerant fermion systems close to a magnetic instability may induce or inhibit superconductivity depending on the nesting wave vector qo for which the instability occurs. If~q o~i s small but finite, triplet pairing is favored and singlet pairing is suppressed as efficiently as in nearly ferromagnetic systems (qo --0). If~q o~i s large, there is a repulsive contribution from backward scattering by which triplet as well as singlet pairings are strongly depressed.The cases of UPt3, CeP13, and some organic compounds are considered.It has been known for a long time that strong spin fiuctuations ("paramagnons'") in nearly ferromagnetic (NF) fermion systems, prevent BCS singlet pairing superconductivity, ' but favor triplet pairing. This result has been well illustrated in the case of the triplet superconductivity of liquid He. The present note considers the effects of a paramagnon-mediated interaction near to a magnetic instability at finite wave vector qo. It is found that if the system is nearly antiferromagnetic (NAF) with a large value of~q o~, then backward scattering from the paramagnon-mediated interaction is repulsive for both parallel and antiparallel spin states, so that both triplet and singlet pairings are suppressed. As the wave vector qo is varied (increasing from zero), the paramagnon effect changes continuously from a qualitatively ferromagnetic to a qualitatively antiferromagnetic character, so that the effective interaction between parallel spina changes continuously from a strong attraction to a strong repulsion.These effects appear relevant for the study of possible mechanisms responsible for the observed superconductivity in the heavy-fermion systems UPt3, CeP13, or in some organic compounds under pressure such as the Bechgaard ditetramethyltetraselenafulvalenium salts (TMTSF)qX.We first briefly recall some known results. In the paramagnon model' for a single parabolic band of itinerant fermions, a strong Hubbard contact repulsion I among opposite spins [ Fig. 1(a)] can induce a magnetic transition when it is strong enough for the Stoner criterion:to be fulfilled. Here I=Imax' (q, O)=IX (qo, O) and X (q, co) is the dynamic spin-correlation function for free fermions as a function of the momentum transfer q and the frequency co. When max X (q, O) occurs for q=qo --0, the system undergoes a ferromagnetic instabili-

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