Time pressure exerts influence on the realization of tone patterns cross-linguistically [1, 2]. In Dinka (West Nilotic, South Sudan), the duration of the syllable—the tone-bearing unit—varies due to utterance-final lengthening and vowel length [3]. The nucleus can be short, long, or overlong; level and contour tonemes are associated with all lengths. Tension between duration and tonal realization represents a major source of cross-dialect variation. Dinka dialects have either three or four tones; we explore several dialects with undescribed tonal phenomena. ‘Sycamore,’ ŋâaap, is specified for a Fall tone cross-dialectally. In the Hol dialect, this tone is realized as a Fall utterance-finally (1a) and utterance-medially (1b). In the Ngok dialect, it is realized as a Fall utterance-finally (2a) but as Low utterance-medially (2b) because of time pressure. Utterance-finally, there is lengthening, which is conducive to the realization of contour tones. Utterance-medially, time pressure increases, final lengthening is absent, and ŋâaap precedes a dissimilar tone target. Hol: 1a) /dèeŋ ǎ-màan ŋâaap deng decl.sg-hate sycamore.sg 'Deng is hating the sycamore tree.' 1b) /dèeŋ ǎ-cí̤ ŋâaap máaan/ deng decl.sg-prf sycamore.sg hate\nf ‘Deng hated the sycamore tree.’ Ngok: 2a) /dèeŋ ǎ-màan ŋâaap/ deng decl.sg-hate sycamore.sg ‘Deng is hating the sycamore tree.’ 2b) /dèeŋ ǎ-cí̤ ŋàaap máaan/ deng decl.sg-prf sycamore.sg hate\nf ‘Deng hated the sycamore tree.’ *Slashes represent surface phonological realization.* Cross-dialectally, contour tones are realized as level with time pressure and dissimilar targets, but as contours with greater sonorous duration and a lack of pressure from tonal targets. This phenomenon may also shed light on the diachronic development of tone in Dinka. Under hypocorrection [4], the neutralization of Fall and Low (2b) could lead to a loss of contrast between tonemes. References [1] Xu, Y. and Sun, X. 2002. Maximum speed of pitch change and how it may relate to speech. Journal of the Acoustical Society of America 111, 1399-1413. [2] Zhang, J. 2002. The Effects of Duration and Sonority on Contour Tone Distribution: Typological Survey and Formal Analysis. PhD. dissertation, UCLA, 2001. New York, NY: Routledge. [3] Remijsen, B. & Gilley, L. 2008. Why are three-level vowel length systems rare? Insights from Dinka (Luanyjang dialect). Journal of Phonetics 36(2), 318–344. [4] Ohala, J. J. 1989. Sound change is drawn from a pool of synchronic variation. L. E. Breivik & E. H. Jahr (eds.), Language Change: Contributions to the study of its causes. [Series: Trends in Linguistics, Studies and Monographs No. 43]. Berlin: Mouton de Gruyter. 173 – 198.