Abstract: Hereford and Holstein-Friesian heifers were grazed on kikuyu pasture in late summer. There were six animals in each breed group. Half of the heifers in each breed were also offered 1 kg maize meal supplement per day. Ruminal fractional outflow rate was estimated using iterative procedures by fitting a two and a multi-compartmental mathematical model on fecal marker excretion data after a pulse dose of chromium oxide by mouth. The first fecal sample was collected by rectal grabbing after 8 h and thereafter at 4 h intervals up to 48 h, at 6 h intervals up to 80 h and finally, one last sample was taken at 96 h. Fractional outflow rate in the rumen using the two-compartment model was estimated at 0.069±0.0196 and 0.063±0.0167 for the Herefords without supplement and on supplement, respectively and 0.090±0.0502 and 0.061±0.0151 for the Holstein Friesians without and on supplement also, respectively. Estimates from the multi-compartment model were 0.056±0.0090 and 0.062±0.0089 for the Herefords without supplement and on supplement, respectively and 0.0607±0.0134 and 0.0559±0.0057 for the Holstein Friesians without and on supplement also, respectively. There were no significant (P>0.05) breed or supplement effects on fractional outflow rate in the rumen.