Using the autoradiographic technique of percent labeled mitoses (PLM), total cell cycle length (Tc) and length of the cell cycle phases G1, S, G2, and M have been estimated for the neuroepithelium and heart of day 10 rat embryos. The Tc for the neuroepithelium was estimated to be 9.45 hr, consisting of G1 of 1.24 hr, S of 6.15 hr, G2 of 1.34 hr, and M of 0.72 hr. The Tc for the heart was estimated to be 13.37 hr, consisting of G1 of 4.30 hr, S of 7.01 hr, G2 of 1.81 hr, and M of 0.25 hr. Comparison of cell cycle parameters in these two tissues indicates that the longer cell cycle in heart tissue is related primarily to an increase in G1. If embryos are exposed to the major teratogenic metabolite of cyclophosphamide, phosphoramide mustard, cell cycle analysis reveals that the S phase of the neuroepithelial cell cycle is lengthened and cells are either slowed or arrested in G2.