For a long time, downstep and H raising have been considered to be two of the phonological processes that are partially responsible for generating the surface F0 contours in languages. Downstep refers to the lowering of the second H relative to the first one in a tone sequence of HLH. H raising refers to the raising of the H in an HL sequence. Recently, Laniran and Clements suggest that, in Yoruba, both phenomena may be viewed as the consequences of regressive upstep, a process by which an L raises an H that occurs earlier [Y. O. Laniran and G. N. Clements, Proc. 13th ICPhSc, Stockholm, Vol. 2, pp. 734--737 (1995)]. Meanwhile, a series of studies on contextual tonal variations in Mandarin and Thai reveals phenomena also reminiscent of downstep and H raising. Due to the nature of the tones in those languages, however, explanations in terms of phonetic mechanisms seemed to emerge. The present paper reports a study that examines both local and long distance contextual tonal variations in Mandarin. It is argued, based on data found in this study, that there are actually two phonetic mechanisms underneath all these phenomena: carryover assimilation and anticipatory dissimilation.