A coupling having a metal-to-metal seal between a male member and a female seal member, the seal being "preloaded" by means of an internal preload mechanism. The male member engages the seal member, forming the sealing relation between the two. The seal member is allowed to float, or slide, longitudinally within the bore of the female coupling member between a first, extended position and a second, compressed position. The seal member is biased by the preload mechanism into its first, extended position. A force resulting from engagement of the male member and the seal member tends to urge the seal member toward its second, compressed position and that force is opposed by the preload mechanism. The sealing relation between the male member and the seal member is thereby enhanced. An annular metal ring seal may be used to provide a sliding fluid seal between the outer circumference of the seal member and the bore of the female.