Mean daily milk yield for 478 Holstein-Friesian cows from two herds, recorded between 1984-1985, and for 218 Jersey cows from two herds, recorded between 1984-1985, were used to estimate lactation curve parameters for Wood’s gamma function: Yn = A nbe-cn.Yn is the predicted average daily milk yield in the nth week; A is a scalar representing ‘starting yield’; and e is the base of natural logarithm. The values of b and c (lactation curve constants describing the rates of rise to and decline from peak lactation, respectively), lactation persistency (s), goodness of fit (R2) and projected 44 week milk yield were estimated. The analysis of variance indicated significant effects (P <0.01) of herd, parity and season on most of the lactation curve parameters in both breeds. The year effects were significant (P <0.05) in the Jersey but non-significant (P >0.05) in the Holstein-Friesian. The use of monthly or weekly milk yield totals to calculate mean daily yield had highly significant effects (P <0.01) on the lactation curve parameters of the Jersey.