The purpose of the present study was to determine the contributions of fundamental frequency (f (0)) and formants in cuing the distinction between men's and women's voices. A source-filter synthesizer was used to create four versions of 25 sentences spoken by men: (1) unmodified synthesis, (2) f (0) only shifted up toward values typical of women, (3) formants only shifted up toward values typical of women, and (4) both f (0) and formants shifted up. Identical methods were used to generate four corresponding versions of 25 sentences spoken by women, but with downward shifts. Listening tests showed that (1) shifting both f (0) and formants was usually effective (~82%) in changing the perceived sex of the utterance, and (2) shifting either f (0) or formants alone was usually ineffective in changing the perceived sex. Both f (0) and formants are apparently needed to specify speaker sex, though even together these cues are not entirely effective. Results also suggested that f (0) is somewhat more important than formants. A second experiment used the same methods, but isolated /hVd/ syllables were used as test signals. Results were broadly similar, with the important exception that, on average, the syllables were more likely to shift perceived talker sex with shifts in f (0) and/or formants.