Effect of nitric oxide synthase (NOS) inhibitors and substrate on follicular development in neonatal and immature rats. Animals in the control, L-NG-Nitroarginine Methyl Ester (L-NAME), S-Methylisothiourea (SMT), L-arginine (L-Arg), or L + S (L-NAME plus SMT) groups were subcutaneously hypodermically injected with phosphate buffer saline (PBS), L-NAME (40 mg/kg), SMT (10 mg/kg), L-NG-Nitroarginine Methyl Ester (40 mg/kg) and S-Methylisothiourea (10 mg/kg), or L-arginine (50 mg/kg) for five (PD5, A), 10 (PD10, B), or 19 days (PD19, C) from the first day of birth (PD1). UF, unassembled follicle; PmF, primordial follicle; PF, primary follicle; SF, secondary follicle; AF, antral follicle. Bars with different letters denote significant differences (p < 0.05).