We utilized templates of defined sequence to investigate the mechanism of primer synthesis by herpes simplex virus 1 helicase-primase. Under steady-state conditions, the rate of primer synthesis and the size distribution of products remained constant with time, suggesting that the rate-limiting step(s) of primer synthesis occur(s) during primer initiation (at or before the formation of the pppNpN dinucleotide). Consistent with this idea, increasing the concentration of NTPs required for dinucleotide synthesis increased the rate of primer synthesis, whereas increasing the concentration of NTPs not involved in dinucleotide synthesis inhibited primer synthesis. Due to these effects on primer initiation, varying the NTP concentration could affect start site selection on templates containing multiple G-pyr-pyr initiation sites. Increasing the NTP concentration also increased the processivity of primase. However, even at very high concentrations of NTPs, elongation of the dinucleotide into longer products remained relatively inefficient. Primase did not readily elongate preexisting primers under conditions where free template was present in large excess of enzyme. However, if template concentrations were lowered such that primase synthesized primers on all or most of the template present in the reaction, then primase would elongate previously synthesized primers.